Okay, a bit of clarification first, to see where I'm coming from.
First, I'm pretty strongly anti-NTR (in the most general sense of the term). I'm not a fan of other characters messing with my LIs once I've committed to making them my LI. I definitely don't want the LIs having sex with anyone else once I've started going after them (and I absolutely don't want to see it as a side-scene or anything). And I consider lesbian scenes to fall into this as much as I do straight sex scenes - I don't want other women going after my LIs without my knowledge/permission (as the player, not the character) any more than I want them being with other men. And I'd generally much rather be going after unattached LIs rather than having to compete with someone else for their affection, or seducing them away from a current partner.
So all that is going to color how I feel about their past history.
That being said, I'm generally pretty chill about what the LIs were up to before I meet/start to seduce them (or before the game starts). I don't need every possible LI to be a naive virgin who's never even seen a dick before (or vagina - I play as female protagonists as well when there's an option to do so) when I meet them. In incest games I'm not hugely bothered by the idea that my mother was with my father before me (so I don't need convoluted weird BS justifications like how she was artificially inseminated or something just to keep her immune from all prior dick). As long as a female LI isn't constantly talking about how much they loved their previous partner or how great the sex was, I don't really care as long as they're with me now.
It's not appealing if the LI in question is a total slut with dozens of prior partners and a really bad reputation, but she doesn't have to be utterly pure and untouched before I woo her either.
I'm not entirely against the idea of winning a LI away from their current partner by being just that awesome if they haven't been intimate for a while, or even straight-up cucking a dude and making sure he knows it (if he's been a phenomenal asshole in the story and it feels like he needs to be punished with righteous fury), but my preference will always be slowly building up relationships with LIs who are otherwise unattached when they meet me, and willing to prioritize a relationship with me over anyone else.