great_scott saidMarriage between two guys hasn't really been legal up until fairly recently, but it could also be because we're guys. Women are basically programmed to want to commit to stuff from day one. They're thinking about their wedding day before they even hit puberty. What's the most common toy for young girls? A fake baby. Guys aren't typically raised in the same way, so I'd say it's generally a guy thing. Maybe the issue is just compounded when it's two guys.
^This. There are biological, social, and legal differences between heterosexual and homosexual communities that account for commitment-phobia of gay men relative to straight men.
I think men (gay or straight) are evolutionarily promiscuous, biologically predisposed against monogamy and towards spreading their genes. Women, meanwhile, seem to be evolutionarily less promiscuous and biologically predisposed -- as maternal caretakers -- to settling down with one partner to protect their offspring.
Add to that homosexuality has been a social taboo until just recently. And on top of that, gay marriage is legally impermissible in most jurisdictions.
Take away from straight males the biological imperatives of their female partners, take away the social pressures and norms of expected heterosexual courtship, and add legal barriers to straight marriage, and I'd bet straight men would be just as "afraid of commitment."
As more gay men practice monogamy and expect it from their partners, and as gay marriage becomes legal, longer-term commitments like marriage should become much more of a norm for gay couples. But I doubt you will ever see gay men getting married in the percentages that straight men do, because of the biological component.
(Does anyone know if gay males in countries where gay marriage is legal marry at the same rate as straight males?)