My female friends tell me it is the news that after 30, the rate of Down's syndrome increases, as well as issues with conception I believe. This is incredibly motivational towards settling down, I hear. A biological clock, whose ticking is getting perpetually louder for all of them.
There is probably more societal pressure for women not to be single either. Bachelors (and excess males) have been around forever, and in a sense, gay men are almost like perpetual bachelors, and society lauds us in a sense. Poor women, when they are single in their 30's (heck, even sometimes their late 20's) people shake their heads, try to set them up with men, market everything from online matchmaking to speed dating, to self help books, and they all imply that there is something so terribly wrong with the fact that she is not married.
On the other hand, we (for the most part) have no biological clock, and the societal pressure of getting married is absent. Heck, you're probably making some tea party member's day by not entering into a monogamous gay marriage.
Lastly, when women encounter children or hold babies, there is a biological urge to have their own and start nesting. Whereas, when I encounter children or babies, I think "Cute, but who cleans up all that shit? And it had better not start crying next to me when we're in a restaurant."