Well I would have to agree that her interactions with Jason are out of character for several reasons, first she just met the guy. If she really is a virgin and that was her first kiss, would she let some dude she did not know from Adam go that far with her. I don't really see it no matter how charming he might be, it is just not something about her character that you have established to the player. Also she has just indicated that she was interested in the MC, and the player has made an effort to show interest in her, yet she still throws herself at a guy who literally just walked into the party, that is very unrealistic, for the love path. I could see it for the lust path as then the MC would have either spurned her advance or shown interest in multiple other women and not much interest in how she felt. But for the love path, the MC is making an effort to understand her feeling and return them.
It also has no buildup for this action, she pores her hear out to the MC the day before, takes a shower with him that morning and in every other regard returns his advances towards her. To have her just allow this guy she just met to go as far as he has just seems out of character, as the character has been presented to the player. A kiss yes, fondling her breasts maybe, exposing her breast in front of the man she just confessed to love, and allowing him to finger her, No.
As for the Fiance getting angry about the MC stepping in, well in this situation you have a complete stranger, taking advantage of the MC's BF from childhood who is clearly drunk or tipsy. In real life I would step in and kick the crap out of the guy. The MC has no idea who he is, the only person vouching for him is a woman the MC literally met yesterday and the MC's BF has not seen in years and neither one really knows anything about. He also just showed up without the MC's knowledge to the MC house and party. Then he started making moves on and sexually assaulting, she is drunk, one of the MC's best friends. If that happened in real life the guy would be lucky to get out of the party alive.