Am I gay if when ever I look at tighty whities I get an erection?

Maybe, maybe not. No two men are the same. Many men get excited when their team scores a touchdown, watching a man ride a bull at a rodeo, seeing a military parade, riding the subway and getting jostled, or anything else that says "being male is great." which it is. Only a few percent of us prefer to partner with another man, but that's fine too. Talk with somebody wise and understanding, and be glad to be you.
Maybe...maybe not. If you have sexually feeling toward people of the same sex, maybe u are gay. Some people are bisexual--they enjoy the sexual comany of both genders. Is the feeling something new or has been there many years and u just never had the courage to ask/let someone else know? Talk to a friend/family, therapist, doc, counselor etc...They maybe able to help sort things out for you. Good luck.