1) your character is
a person who has sex in front of an audience in an illegal brothel for money -
but you don't want to be disloyal to Ian
I'm aware that any attempt at morality is going to be undermined by other behavior that is unavoidable. That's just the nature of the scenario. But, being a willing participant in an illegal brothel's sex pageant is irrelevant to the question of whether you should backstab your lifelong friend or not.
2) Ian is a
serial cheater, who disrespected his girlfriend over and over and over;
Which sucks for everyone involved, but it's no excuse to stoop to the lowest common denominator.
Probably worth to keep in mind that it's more of Mina reacting to discovery that she's been repeatedly cheated on by her supposed boyfriend. They're as good as done by that time, even if Ian isn't informed about it (or you can even choose to pretty much inform him about it, by notifying him that Mina has found out about him being a cheating dipshit)
3) Mina is NOT cheating on Ian - she's about to dump him, and she will do it in a a few days (I don't remember when it exactly happens right now) anyway; and you see Ian having sex with quite a few women (Carnations, Club girls, his maid, the hairdresser) before and after Mina dumps him;
Still, I'd like to hear that they're officially broken up before getting involved.
4) the game makes you choose between being best buddies and being an asshole with Ian;
So far I've pretty much universally shut down Ian's bad behavior. So my "best buddy" status is probably in jeopardy, but in this one instance I've chosen not to damage the friendship.
5) are you aware that this is an erotic novel? your aim is to get laid.
Sure, but it's also about roleplaying and I am behaving in the way I wish to. Behaving in /any/ fashion is going to open some doors and close others. For instance, one of my favorite scenes plays out very differently depending on how you behaved during a sex scene earlier in the game.
Hana? your argument makes no sense. Their agreement is: he is allowed only in the club and because of job obligations. If Edwin breaks this rule, a game variable adds cheating points: so it's only up to you only if you cheat on Hana while she is your girlfriend.
I've only ever read through that scene once, and I chose to not have a relationship. If there's some discussion of terms and allowed extracurriculars in the girlfriend route, then I haven't read it yet. But, iirc when she asks, Hana is unsure of whether she would be comfortable with Edwin sleeping with other women for club business. To me that seems like it wouldn't actually work out. Plus I'd like our relationship to be exclusive on /both/ ends, and if I can't promise that then I have no business being in a relationship. Also morality based on game variables is metagaming which is lame.