Part of the problem is that we simply want a safe space for ourselves. In the few cases that we have indeed excluded people, believe me it's because they've deserved it. We wouldn't do it because "eww straights are icky and they're all eeevil."
If you can come in to our space and be respectful of the people there and the reasons they are there, then fine. But if you're only there to try to make yourself look cool and trendy, without understanding that this is our space, then yes there'll be some resentment.
Out in the world, LGBTs are the minority. Even as well accepted as we may be in some locations, surely you can understand why we would want a safe space that is just ours, where we don't have to worry about potentially upsetting other people just by being ourselves, and how we would be uncomfortable with too many non-LGBTs invading that.
If you look at the gay scene in Manchester, UK, it's barely gay anymore. Over the years it became the trendy place to be, where straight girls could go to party without being hit on by straight men. Then of course, the straight men followed them because that's where the girls were - at the gay bars. Eventually there were so many heteros there that gays stopped going.
In too many cases in less accepting locations, the local gay bar is the only place a LGBT person can feel safe and free to be themselves. They provide a vital resource.
And sadly gay bars aren't even always a safe space from aggression. There was the case recently that two guys went into a gay bar in New York - the legendary Stonewall bar itself in fact - just so that they could beat up some gays. They beat the crap out of some poor kid in the bathroom, and that was their entire reason for being there. When even the spaces we've carved out for ourselves aren't safe, you can understand why we'd be skittish.
