Hmmmm, I'm not sure I'm an example that supports the HD/oral connection. I'm classic HD. In terms of oral, I can run the gamut... to absolutely out of the question to obsessed.
For me, it's not just the acts (no matter which end you're on) there are other factors. While I agree with some of the earlier points (like recent shower, reciprical pleasures, adding foods/drinks) those are all secondary. For me, the first thing is 1. do I like the person this penis is attached to enough to enjoy giving this much pleasure? if not, no way. 2. do I trust and like this person enough to let them give me this much pleasure? If I'm not sure, no way. It makes me too vulnerable.
Oh, and the other question was about how to ask? When things are right, I don't have to be debated or cajoled-- I'm already there. When they are not right, there is nothing anyone could say, ask to persuade me. And I'd never explain it, or any of the little how to's or boundaries to anyone who wasn't suckable.... back to that trust factor.
Never try to teach a pig to sing. It wastes your time, and annoys the pig.