The short answer is that when a men acts in a way that is sexually suggestive it's...'threat' isn't the right word...It's a provocation. Whether the man intends it to be or not. It's an 'active' display, so to speak. Thus, seeing men act effemininity or sexually towards them (and not the 'gay bravado' that straight men do around each other with is in fact reinforcing how straight they are) makes other men angry.
For women, on the other hand, this is not the case. Women, therefore, have much, much more freedom to be sexually suggestive in situations that men would never allowed to be without facing disgust and outright contempt. And it doesn't bother women or gay men because a woman doing such things isn't a provocation. It's passive, not active.
And I must question why do stright men get angry. And the only answer I can come up with is because it makes them feel uncomfortable. They feel, for lack of a better word, "violated" in terms of their mental space. The argument that I've seen posted in various parts of the internet, is that straight men are uncomfortable with the idea of gay men because they know (think) that gay men will treat them the same way [straight men] treat women.
I don't think it should be a problem that a male NPC flirts with, or comes onto a male PC, so long as you can decline such an invitation. Preferably, they'd give you options on how to do so, whether you want to be nice about it or not (it's a game, if people want to be homophobic in their game, then so long as it is just one of many options, I really don't have a problem with it).