It reflects reality, so why not? 
Though I have to say that sexuality (or belonging to a minority at all) should never be a character's defining or primary character trait.  Imagine if there was a character created for the sole reason of being heterosexual...

I think the very question of whether characters should be gay is in itself a bit of an insult to the gay community. Why use the word should? That implies that if the result of the argument is that they shouldn't, then there would never be any gay comic book characters. And that would be a particularly sad place for the art form to be.
 
If a creator has a story that involves a gay character then it should be included. I see absolutely no reason why it shouldn't be. And if you are are offended by it by reason of religion then you have a great get-out-clause - DONT BUY IT! You see the beauty is in choice. If you don't like it, don't buy it. I hate reality tv, so I don't watch it. And therefore it doesnt bother me. If gays get you so hot and bothered then avoid the comics with homosexual characters. 
 
And to address the point about disliking storylines that use gay characters for the sake of it - I agree. It is used as a gimmick, but one that can offend gay people (as well as the up-in-arms homophpbes).