My experience is that men usually didn't require or even want a romantic commitment. Most of the guys that I know are more interested in sex and not formal relations... while women are usually interested in relationships, rings and dinners hahah of course there are people out of this spectrum but this is my experience. This one of the reasons that make me so not into a relationship with men.
Of course there's also the fact that society accepts more straight relationships, so I think that's other strong reason that I'm not romantically attracted to men, the truth is that I really want to avoid conflicts and this things and if I can do it why wouldn't I do?
But in resume, it's not a so rational thing, I tried two rationalize it here because you asked about this aspect of bisexuality, but I don't usually think about it. I'm just not interested in relationships with men if I can have it with women, and in my case I'm even more sexualy attracted to women than to men, so I think it make sense
