Bisexuality is generally the most common sexuality to deal with this, I think. It's because there are so many people who use bisexuality as a way to gradually phase in the idea that they're gay, and also so many ****ty 15-year-old girls who say they're bisexual so they can kiss each other to turn on boys, that people start to question whether the sexuality is legitimate at all. It's a case of some people ruining it for everybody.
It's not right, it's absolutely not OK, but I do understand why people might think that. It's just a matter of educating them, as frustrating as that might be