Are you gay if you like to be butt banged by your girlfriend and love to be spanked and loves seeing boys in their underware but do not like boys?

You have answered your own question... "by your girlfriend"... "do not like boys." If you were gay you wouldn't have a girlfriend, as you would be sexually and emotionally attracted to boys. You are simply a heterosexual (straight) guy who enjoys having his prostrate stimulated by having his girlfriend penetrate him. As a teen the sight of anybody (male or female) in their underwear or naked can be sexual stimulating, which is why many boys get an erection in the changing room or showers at school. It does not mean that you are gay, just that you are easily aroused. Spanking has nothing what so ever to do with being gay or straight, it is simply a fetish activity that some people enjoy. Perhaps the interest in boys underwear is just in the underwear and not actually in the boys who are wearing it? If it is also a fetish it might be an idea to buy some and persuade your girlfriend to wear them, it could be a major turn on.