So, with regards to sex specifically, all stats are equally useful in accomplishing it with any girl (though note that in the case of Jean, her displayed stats are usually lies as I covered
). The basic formula to check if a girl is willing to have sex checks her combine stats (Love + Obedience + Inhibition) alongside a pair of modifiers, and compares the result to 1400. If the final result is at least 1700, she really is into it, if it's between 1550 and 1699 (inclusive) she is into it, and if it's between 1400 and 1549 (inclusive) she'll go along with it. As for the modifiers that are relevant, they are a value called Tempmod and a value called Taboo.
Tempmod is based on the following (and possibly some previous events since it isn't set to 0 at the start of adding all of these):
Edit: I found the main section regarding Taboo Value, though I can't rule out any other parts influencing it. Aside from the last point, only the first valid option takes precedence:
Also, from what I can tell, after these checks Jean will set her Taboo level to 0 so long as she is not restricted from mind whammying people to make them forget seeing her do stuff.
Anyway, when determining if the girl will be willing to have sex, after adding up her stats, you then add 10 times the Tempmod value, and subtract 50 times the Taboo value, then compare it to the previously noted values to determine if she'll agree.
This is incorrect. The bikini pieces are actually quite poor as far as how much benefit they give. Mind, gifts have stat breakpoints that impact how effective they are:
- Dildo:
If the girl has at least 800 combine stats, it increases all three stats by 30, up to the cap of 1000.
If the girl has combine stats between 500 and 799 (inclusive) the girl will gain 10 in each stat, up to the cap of 1000.
If the girl has less than 500 combine stats, she will refuse the gift, and if it's the first time you've tried giving it to her that day, she will lose 20 love, and gain 10 obedience and 20 inhibition, up to a limit of 200 for each of those two stats.
If she already has a single dildo, the girl will accept a second, but without any stat changes. She will refuse a third dildo, again with no stat changes.
- Vibrator:
If the girl has at least 700 combine stats, it increases all three stats by 30, up to the cap of 1000.
If the girl has combine stats between 400 and 699 (inclusive) the girl will gain 10 in each stat, up to the cap of 1000.
If the girl has less than 400 combine stats, she will refuse the gift, and if it's the first time you've tried giving it to her that day, she will lose 20 love, and gain 10 obedience and 20 inhibition, up to a limit of 200 for each of those two stats.
If she already has a vibrator she will refuse a second with no stat changes.
- Buttplug:
Listed in the code as a gift you can give, but has no method of gaining one, has no requirements for a girl to accept it aside from not already owning one, and causes no stat changes when given. So even if you cheat to put one in your inventory, there's no point.
- Dazzler and Longshot book:
Always increases Love by 50, up to the cap of 1000. However, if the girl already has 600 Love she will react far more favourably to getting the book.
- 256 Shades of Grey book:
Girls other than Jean will increase Obedience by 50, though if the already have at least 500 Obedience they'll be more interested in their reaction.
Jean instead will comment on "what a weekend that was" before she her reaction changes based on the previously noted check for 500 Obedience. If she does have 500 Obedience, she'll gain 5 Love, up to a limit of 500, then gain 3 Love, up to a limit of 900, then gain 5 Obedience, up to the cap of 1000, before asking if the player wants to try some of the stuff in the book. If she has less than 500 obedience, she'll instead lose 5 Love, gain 5 Obedience, up to the cap of 1000, and lose 5 Inhibition. Either way, she will then gain 10 Obedience, up to the cap of 1000.
- Avengers Tower Penthouse book:
Always increases Inhibition by 50, up to the cap of 1000. However, if the girl already has 600 Inhibition she will react far more favourably to getting the book.
- Nighty:
If the girl has at least 600 combine stats, she gains 40 Love, 20 Obedience, and 30 Inhibition up to the cap of 1000 for each.
If the girl has less than 600 combine stats, she gains 30 Love, 20 Obedience, and 20 Inhibition up to the cap of 1000 for each.
- Corset:
If the girl has at least 1000 combine stats, she gains 20 Love, 20 Obedience, and 10 Inhibition up to the cap of 1000 for each.
If the girl has between 700 and 999 combine stats (inclusive) or is Jean and does not have the 1000 combine stats, she gains 15 Love, 20 Obedience, and 10 Inhibition up to the cap of 1000 for each.
If the girl has between 600 and 699 combine stats (inclusive), she gains 10 Love, 25 Obedience, and 15 Inhibition up to the cap of 1000 for each.
If the girl has less than 600 combine stats and hasn't already refused a Corset or Bra once that day, she refuses the Corset, loses 10 Love, and gains 10 Obedience and 20 Inhibition up to a limit of 200 for each.
- Lace Corset:
If the girl has at least 1200 combine stats, she gains 25 Love, 30 Obedience, and 20 Inhibition up to the cap of 1000 for each.
If the girl has between 1000 and 1199 combine stats (inclusive), she gains 20 Love, 30 Obedience, and 15 Inhibition up to the cap of 1000 for each.
If the girl has between 800 and 999 combine stats (inclusive), she gains 20 Love, 20 Obedience, and 25 Inhibition up to the cap of 1000 for each.
If the girl has less than 800 combine stats and hasn't already refused a Corset or Bra once that day, she refuses the Lace Corset, loses 10 Love, and gains 10 Obedience and 20 Inhibition up to a limit of 200 for each.
- Lace Bra:
If the girl has at least 1000 combine stats or is Jean, she gains 30 Love, 20 Obedience, and 30 Inhibition up to the cap of 1000 for each.
If the girl has between 700 (or 600 for Emma) and 999 combine stats (inclusive), she gains 25 Love, 20 Obedience, and 20 Inhibition up to the cap of 1000 for each.
If the girl has less than 700 (600 for Emma) combine stats and hasn't already refused a Corset or Bra once that day, she refuses the Lace Bra, loses 10 Love, and gains 10 Obedience and 20 Inhibition up to a limit of 200 for each.
- Lace Panties:
If the girl has at least 1100 combine stats or is Jean, she gains 30 Love, 20 Obedience, and 30 Inhibition up to the cap of 1000 for each.
If the girl has between 800 and 1099 combine stats (inclusive), she gains 25 Love, 20 Obedience, and 20 Inhibition up to the cap of 1000 for each.
If the girl has less than 800 combine stats and hasn't already refused Panties once that day, she refuses the Lace Panties, loses 10 Love, and gains 10 Obedience and 20 Inhibition up to a limit of 200 for each.
- Stockings and Garterbelt:
The girl always accepts the gift if she doesn't have it already and gains 5 of each stat, up to the cap of 1000.
- Pantyhose:
The girl always accepts the gift if she doesn't have it already and gains 5 of each stat, up to the cap of 1000.
- Knee Stockings:
The girl always accepts the gift if she doesn't have it already and gains 5 of each stat, up to the cap of 1000.
- Bikini Top:
If the girl has at least 1200 combine stats, she gains 20 Love, 10 Obedience, and 10 Inhibition up to the cap of 1000 for each.
If the girl has between 900 and 1199 combine stats (inclusive) or is Jean and does not have the 1000 combine stats, she gains 20 Love, 10 Obedience, and 5 Inhibition up to the cap of 1000 for each.
If the girl has between 700 and 899 combine stats (inclusive), she gains 10 Love, 5 Obedience, and 5 Inhibition up to the cap of 1000 for each.
If the girl has less than 700 combine stats and hasn't already refused a Corset or Bra once that day, she refuses the Bikini Top, loses 5 Love, and gains 5 Obedience and 10 Inhibition up to a limit of 200 for each.
- Bikini Bottoms:
If the girl has at least 1200 combine stats, she gains 20 Love, 10 Obedience, and 10 Inhibition up to the cap of 1000 for each.
If the girl has between 900 and 1199 combine stats (inclusive) or is Jean and does not have the 1000 combine stats, she gains 20 Love, 10 Obedience, and 5 Inhibition up to the cap of 1000 for each.
If the girl has between 700 and 899 combine stats (inclusive), she gains 10 Love, 5 Obedience, and 5 Inhibition up to the cap of 1000 for each.
If the girl has less than 700 combine stats and hasn't already refused Panties once that day, she refuses the Bikini Bottoms, loses 5 Love, and gains 5 Obedience and 10 Inhibition up to a limit of 200 for each.
- Blue Skirt:
If the girl has at least 1000 combine stats, she gains 20 Love, 10 Obedience, and 10 Inhibition up to the cap of 1000 for each.
If the girl has between 800 and 999 combine stats (inclusive), she gains 20 Love, 10 Obedience, and 5 Inhibition up to the cap of 1000 for each.
If the girl has between 600 and 799 combine stats (inclusive), she gains 10 Love, 5 Obedience, and 5 Inhibition up to the cap of 1000 for each.
If the girl has less than 600 combine stats and hasn't already refused a Skirt once that day, she refuses the Blue Skirt, loses 5 Love, and gains 5 Obedience and 10 Inhibition up to a limit of 200 for each.
For the highest stat gain when giving it to the girl, the dildo and vibrator are the best choices. However, Any of the options that allow you to fail to give them to a girl also can be used for a net gain in stats by failing to giving them to her once a day to increase Obedience and Inhibition at the cost of Love, up to a cap of 200 for the two increasing stats. How useful that trade is may depend on how easy it is to boost Love for the girl. Also, if trying to maximise the stat gains from gifts, it's best to wait until you meet the requirements for the best tier of said gift.
Outside gifts, there are three other main ways to speed up the stat grind. Doing new sexual acts with the girls (each time you do an act for the first time, as well as seeing the girls exposed breasts or crotch for the first time, you will get a notable boost to stats). Going on dates with girls can give a boost. Doing the Halloween event in the evening allows you to flirt multiple times with the same girl in a single time slot (you are only limited by the progression of time after each action as eventually you will hit the end of the party).