because we actually want to see the woman get fucked, generally speaking, and like 50 scenes with one girl we're attracted to, beats like, 2, when there's a dozen unfleshed out girls.
plus the whole corruption thing is kinda hot for some people.
or even just, the usual 'dudes have a harem/multiple romance routes', a bit meh. girl turns into a total slut and fucks everyone, bit more flavor on those bones.
but, as most things, it's a bit subjective. hell, i don't recall many ntr dude games (as in, the guy cheats on his girl), but the girl cheating seems done a ton (even when the lead character is a guy, hell, there's one where the dude's supposed to pick teh chick he ends up with, and it's still ntr of all of them getting fucked without him if he takes too long or chooses someone else), because the focus of the girl getting fucked i guess is inherently more interesting to the average porn fan. or at least, the ones into ntr i guess, since that's a bit niche.
