As a queer woman in lesbian bars I've experienced the opposite of your story. All I can say is that straight guys, I could usually tell why they were there. Some just lived in the neighborhood, it was their local watering hole, and never bothered anybody or hit on anyone. Some came along with their lesbian friends. The only time I had a problem with straight guys in a lesbian bar is if they were hitting on the women there. So, if I'd met you in one of these bars, I'd be ok with you. I much prefer mixed bars. More variety, more open-mindedness. And the mixed softball teams are way more fun. The one thing about straight women going to gay/lesbian bars is that lesbians have a much harder time figuring out which women are straight, and who is gay. You can't always tell just by looking. (Male sexuality, I've found, is much more overt, direct, while female sexuality is more subterfuge-like.)
Yeah, don't get me wrong, I may not have put enough emphasis on this, but I only ever felt out of place when I was alone at a very aggressively segregated lesbian night. If anything I've found it easier to talk to women at lesbian bars because there isn't a weird sexual cat and mouse game going on and I feel like I can just put on a casual persona and chit chat. I don't think that I would necessarily seek out a lesbian bar without a lesbian friend (actually I know that I wouldn't,) but if my neighborhood bar happened to be one I'd probably feel comfortable enough getting a drink once in a while. The sexuality of the clientele would never really be me first concern anyway.