Does he worry about losing his erection?

That's the only thing I can think of.

Have you ever complained about it in the past? Does he have built up resentment wrt to it?

I know that my H has several times in the past alluded to not wanting to do it anymore and I have finally given up. If he wants to, fine, but I'm not going to ask or hint or even accept his offer enthusiastically. He's made it clear that he's now iffy about it, and so he has essentially ruined the experience for me.

But..it's a small portion of a whole that is pretty darn good, so that is not the gigundous complaint that it appears to be.