Some gay men, on those online dating sites, have it listed that they want kids. Is this a common yearning for gay men?
I personally don't want kids for a myriad of reasons:
They are too expensive, especially if raised from infancy.
They are too emotionally taxing - worrying about their welfare more than your own or even your lover.
I don't have the nurturing spirit to raise a child. Rather, I have an experimental spirit in that I'd try various methods of shaping how my child mentally and physically develops in a more cold way. there isn't a book on parenthood, but I can say for sure that's not a good way to raise someone. This is the second-biggest reason for me, next to the cost.
Did I mention they're expensive?
I can't tolerate kids for even 10 minutes, right now. Not even family. They try my patience. Ever seen those parents with 3 kids and how unhealthy they, themselves, look? That'd be me with 1.
I'm not ready, nor do I feel I will ever be, to father a child (but who is, at 21?

).
Kids are EXPENSIVE! And, sometimes, the 18+ years investment doesn't pay off. :x
That's BIG chunk out of my life. You're devoting MUCH more of yourself to this person than you would and/or could even do for your partner.