The answer lies in the question you asked. The reason there's only a handful of homosexual male athletes is because it isn't as common, so it's probably more difficult for them to be bold about their sexual preference.
Homosexuality among female athletes is quite the common ground. This makes it easier for them to open up about their sexuality.
There are many more homosexual athletes, whether they be male or female, than we know about. They're certainly there, they just don't talk about it.
To say there are more lesbians in sports is just an assumption. It's a sad reality that women who are physically strong, outgoing and assertive, tend to be labled as "manly", or "dykes". As a society we always celebrate the strength of our athletes, as long as they fit within the bounds of our established comfort zones. I think this says more about our assumptions on gender roles than it does about homosexuals in sports. I cannot understand why anyone would think that sezua preference has anything to do with a persons ability to perform well at sports. But maybe I am in a minority in this belief and that is one of the reasons many famous athletes don't mention their preference. Also I think it has been a bit easier for people to accept gay women in general. It doesn't offend their collective sensibilities, it's not as "vulgar" to them maybe? Perhaps I got a little off topic here, sorry, hope my answer made sense.