I was going to disagree with you at first, that it's not so bad. But then I went through my list of swinging/sharing games, and a good chunk of them have at least one sleazy/creepy/douchy guy.
My guess would be that there are several factors at work here.
1) Playing with the NTR feelings. IMO, the the most obvious one, especially if there's an NTR route. It's for the NTR audience, even if it's a sharing game. See, MC/player, your girl is eager and willing to fuck a hobo, an asshole, a big black dude, etc., even though technically it's you who let her. Also, making a nice and respectful lover on the NTS path, who would at the same time cuck the MC on NTR route is somewhat hard. That would be almost 2 different characters.
2) It's what players want. Of course it's all conjecture, since I didn't make a poll or anything. But I feel like a lot of players, even in sharing games, don't want their girl to have any kind of emotional attraction with her lover (I don't like the word "bull") and thus his role is that of a human dildo. If they do anything else, players feel like it's going into NTR territory ("he's becoming her bf"). Sure, you may say it doesn't mean he has to be an asshole. And I agree. But it often means he has no real personality and thus can't really be "likeable".
And because players don't want to be "threatened" by the other guy, it's easy to make him have unlikable features, to make sure he's not the "boyfriend material". Make him rude, unattractive, a playa, some random hobo. Unlike "you" who is kind, supportive and caring to "your" gf.
3) Some players want drama in the relationship and wholesome swinging/sharing bores them. They want potential conflicts between MC and FMC. So let's introduce an antagonist.