If I don't want to get caught up in the drunk-straight-girl-who-calls-herself-bi crowd I stick to "I'm an equal opportunity skank". If you say it right people will laugh and you don't have to label yourself.
"A guy that's sucking cock at 18 will be still be sucking cock at 28, 38, and 108"- not true. My boyfriend used to be a rent boy and was really upset and confused when he started to become attracted to women- he was comfortable with his gay identity. But now he's pretty much straight, so it does happen.