It's not just guys.
Once I went to a straight bar with a group of friends. We were out of chairs so I was squatting on the floor for a good 10-15 minutes having a conversation with a buddy, when my knees got tired. As I stood up I accidentally bumped a lady and knocked the drink out of her hand. Quickly I apologized and offered to buy her a replacement drink.
She accused me of doing so on purpose, so as to have an excuse to buy her a drink. I explained this was clearly not the case, being gay and completely unaware of her presence. A friend of hers sitting at the table nearby, snorted and laughed aloud.
I was then told to get out of her face before her gigantic boyfriend came over to beat the crap out of me.
Mind you, she was probably 25 years older than me and nowhere near what I would define as "good looking."