Women can be categorized into 3 tiers.As already said, she only goes for Jeremy if Ian shows no interest in her. How dare she dates guys other then me! In particular, she jumps on Jeremy right after she sees Ian kissing Emma. If you/Ian expects her to be faithful while you/Ian can fuck around, you are beyond hope.
Anal: She even gives it a try and it doesn't work physically. She is only pushing for open relationships and threesomes because she thinks its what Ian wants and she wants to be as exciting for him as possible. If you deny those options, she is happy to carry on.
Emma doesn't need Ians permission. Ian just recommends her to Jeremy because he had good experience with her and is a bro to Jeremy. Jeremy got curious, asks her and she simply agrees.
Holly is the best wife material out of all these terrible options in this game (Our Red Flag), because she barely has meaningful flaws and is sort of perfect and a favorit of Eva. She wants to please and has issues with men, both very convenient flaws that the player is very happy to correct. I tend to prefer other girls because they have flaws, issues and aren't at Ians whim every second, but have their own ambitions and ideas of a good relationship. Threat them like trash and they call you out on it, defend themselves and may even break up. With Holly, you are on easy mode. I'd like her second book to fail and see how she takes that loss, but considering how well she is liked by everyone, it probably won't happen and she stays successfull in every playthrough.
Keeper, Sleeper, and Sweeper.
Keeper = Good Wife Material / Long term committed relationship / Start a family.
Sleeper = Fuck buddy / One Night Stand / Friends with benefits
Sweeper = Avoid romantic relationships as the juice is not worth the squeeze.
Alison is definitely not in the "Keeper" camp. She makes a decent "Sleeper". She is a fine "Practice Girl", but not wife material at all.
Emma borders on the "Keeper" camp, but she too is a "Sleeper"... I mean if you were a homeless guy or a junkie, she would be top tier super keeper.
Holly is a Keeper all the way. She is far beyond any other girl in the game in the Keeper camp. She is practically an angel. She is beautiful, and doesn't even know she is pretty. She is the smartest girl in the game by far if you actually listen to them when they talk. She is very kind and caring. She has high levels of empathy. She is nurturing and willing to help teach others. She is not very judgmental at least not in a negative way of others. She doesn't get by on her looks. She has an immense amount of creativity as she is a best selling author. She is polite and friendly. She seems to have a submissive side. She has some loneliness in her heart and she wants to reach out to someone who will love her. She isn't fat. She doesn't have a high body count.
Holly has her kryptonite. Ivy of course knows all of this, and Ivy is her poison. Ivy is the biggest sweeper in the game too. She has all the whore attributes, plus the drugs, and lying and sneaky shit. Pairing the best girl with the worst girl was definitely intentional.
There are justifications for Alison's actions of course as you have mentioned. She just seems to eager to hop in bed with someone new, and it seems like she doesn't care who.