I suspect it's a way of negating another man, and thereby denying what is manly about him. It's an insult that goes against a primary visible difference between men and women. It also speaks to the fact that, to some men, being a woman is the worst thing ever. I'm pretty sure there's a sprinkle of homophobia in there, too. (This is right off the top of my head.) - Ha3rvey (on hiatus)

I've seen observations like yours before, and I would say more or less what Ha3rvey said. Insulting other men by likening them to women's body parts is a way of reinforcing the belief that women are inferior. And it also reinforces the idea that there is something wrong with people who suck cocks, which blurs the line between misogyny and homophobia. Some men (and women) may not realize they're doing this if they haven't stopped to analyze how the words function in our culture and why they're used as insults. - John (bird whisperer)

The underpinning of homophobia is misogyny. That's why in some cultures the man who tops is still considered heterosexual and it's the man who bottoms that is homosexual - because he is taking the "woman's" part. Women suck. In both senses of that word. Women can't throw, can't drive, can't think logically, etc. etc. It's all a vast web of mythology that's part of the world's oldest oppression. Before racism, before homophobia, there was patriarchy. - Spidra Webster

I had wondered if it was a sort of rebellion against what men felt was controlling them. Obsessed by it? Then make it seem unimportant and ridiculous, even distasteful. - MiniMage

I think most -isms come from fear. Not strong fear, but enough to cause uncomfortable feelings and insecurity. If you fear something, tearing it down is the only alternative to accepting that you don't understand it and trying to change that. Bitch is one of the most popular insults from what I can tell. Women are bitches when they are assertive, strong, and self-confident. Men are bitches when they complain, are uncertain, or are hesitant. Women are usually giant/massive/huge bitches and men are little bitches. It doesn't make sense to apply one word to opposite things, other then those qualities are less socially acceptable to be associated with who they are associated with. (I'm sorry if this doesn't make sense, it's bedtime.) - Heather

It's no coincidence that calling a man a "bitch" was an insult that started as basically calling him a woman. Because that's insulting. Men will call each other "woman" or "girl" or "bitch" as an insult. The way that insult has spread has been bugging me for years. I don't like it when women get called it and I really don't like it when men are called it as a way that saying they're less than - just like a woman. - Spidra Webster