Two reasons :
1- Mimetism
When you hang out with gay people, a lot of gay people talk this way. Without really realising it, you just take a few new habits given by your surrounding.
Just like the way your accent becomes less obvious when you've been in a new place for some time now, but returns when you spent a couple of days home.
When you hang out with people, you begin to look a little like them.
Straight people hangin' out mostly with gay dudes (a rare sight, but I know a couple of them) talk that way. Gay people hanging mostly with straight dudes got a slighter "gay talk".
2- Straight men actively try not to talk that way
Because they don't want people to think they are gay.
Openly gay people won't care, why try not to look gay when you are? So you just speak the way you like.
Even without hanging around with gay people, some people will naturally speak with a higher pitch, or with more so-called "feminine" manners.
Straight people who notice they are beginning to speak that way will try to correct that. Gay people would just be themselves.
The addition of both these reasons explains why openly gay people hanging a lot in gay bars/clubs/cruises... are more prone to talk that way
One last reason maybe?
When you hear someone "talking gay", are you reeaaaaally sure he's a homosexual?

When you hear someone speaking normally, are you reaaaaly sure he's not gay?
