i don't think star trek fans honestly care unless they rewrite the character to make Spock gay too, then I can see them crying foul over it. Not that there's anything wrong with being gay, as I think everyone has the right to live however they choose.
Besides, just because the man is gay, it doesn't mean that he can't play the character. Hell, look at Sir Ian McKellan. He's gay, and openly admits it too, but nobody considers it a big deal when he plays the infamous Magneto, a character known to be straight in comics. So I don't see this as a problem at all, as I think it's more of the character he plays in the Star Trek films they would be more concerned about, but not the actor's lifestyle. After all, star trek fans still love George Takei, and he's openly gay, so there you go.