We've considered this issue at length many times.
I tend to think that it does have something to do with learned gender roles.
A woman's
cuddle instinct isn't confined to men only. She can safely express her emotions more readily. A man is instructed to maintain his masculinity from an early age and therefor isn't as inclined to
cuddle with a guy at all.
As for the dominant partner idea; In some relations, there is no dominant partner -- there is a mutual pairing. So, the dominance of masculinity is not an issue. But, men are taught that they are to be the dominant partner (lest they be called wimp) -- and therefor in a MM bi-sexual encounter, perception is that one often must be "the reciever" and therefor the corrolary to the female reciever.
Considering an MM encounter though -- if both men perform oral sex on the other then which is the dominant partner? It isn't required in the mutuality that either be dominant. If one man takes the role of
bottom then he has clearly accepted the traditionally feminine role while the other maintains his traditional male role. Yet, if they take turns at the
bottom role then there is only a temporary dominance issue.
It seems to me then that the dominance of role issue is one of perception and not practice. It's an issue too, that many men who are not bi are concerned that they will have their
man card taken away if they accept the very role that they prefer their women to accept.
There is that further concern among some men that MM sexuality must always include either providing oral sex for the other man or (forbid) recieving anal sex. I think that many men have heard from we ladies enough about anal sex that they're affraid of it.

And then, there is the safety issue there....
I think too that many men are concerned about how their SO will respond to their desire to be with a man. Unless the woman has made it clear that she finds MM stimulating to her -- he will be concerned that she will think he is a
wimp. For the most part, media and men in general have made it clear that two women is their No.1 fantasy -- we ladies have little to worry about if we express an interest in women, we've been taught that "he'll think that's hot" while he's been taught that if he has sex with a man then we'll think he's effeminate.
Maybe it's a function of media and an unlcarified gender role issue. I personally think that any man that wants to have anal sex with a woman (me) should recieve anal sex first. He might even get some ideas on how to do it even better.