How long have you been going out with your bf and you've already discussed that? I would say, the other person's butt, unless you're both men, is off limits until all other areas have been exhausted, so to speak. Then you pretty much spring it on them when they're in a more vulnerable position and hope they thank you, not slug you. That's how I remember it anyway. Let me see if I can find my Rosetta Stone diary back from when I was still doing it...