There are 3 systems involved in this discussion groping, flirting and the sex system.I am not sure if mod and bugfix has anyhing to do with being able to do it (probably not since as far as I know it does the same thing 20 sluttiness and 25 love), but after you get that 20 temporary sluttiness(might be able to start it sooner, but I like number 20) you go to groping (you will have to do it 3-5 times) since that will eventually start sex scene... well groping scene that you can change into anything you want if girl agrees or you are able to convince her. It always goes groping - fingering - vaginal, obviously not in one session and unlocking other options seems optional. The reason why you are able to do it is because temporary sluttiness doesn't all disappear over night and it does slowly change into permament one.
Edit: Besides I am not arguing about if flirting/groping works, you are the one who brought it up. I am just saying that you don't "need" serums to build permament sluttiness after 20.
If you start with flirting and select the continue option it moves you into the sex system.
If you select just flirt it only uses the flirt system.
If you group past the point of grabbing boobs and continue it moves you out of the grope system into the sex system.
If you move on to groping, kissing, fingering.... you are no longer in the flirt system.
I can't say if mods make a change or not. I don't use them.
What it sounds like is you are failing to recognize that 3 systems exist and just lumped it together as the same system because you started off with it.
If you use either of the two routes into the sex system then yes you can raise sluttiness above 20. However, flirting alone won't raise you one bit once you reach 20 sluttiness. That isn't something that is arguable it's black and white written in the code for anyone and everyone to see. I gave you the file and line in the prior response. If you want to continue arguing I won't have fun arguing with the file instead. If you don't find that line of code in your file whatever mod you may be running changed it and that is why you can do something different.