I think it's pretty normal. When I meet someone, I tend to assume they're straight, too. Until I know, I'm pretty inclusive when I talk ("So do you have a boyfriend? Or a girlfriend?") but in my mind, they're straight. Every single person I meet assumes I'm straight, unless it's in an explicitly pro-LGBT environment. I'm fine with that. If it does annoy me, I set them straight. It only really gets to me if they're also LGBTphobic.