Um, unless eHarmony is getting federal funding... why the hell not? It's called freedom of association, last I checked.
In any case, eHarmony doesn't disallow gays to go the site. They can go to it all they want. But it would be kind of silly for them to send them their money when this particular website is not catering to their interests.
There are plenty that do. You get about 4.5 million hits on Google for "gay online dating".
I would just about kill to find a lawyer who would take this case -- I want to sue manline.com because I went there to find a heterosexual man to date and there WEREN'T ANY.
Because it's a site for gay people.
Just like eHarmony is a site for hetero people.