If they can't or won't engage with the topic at all — they deflect, freeze, change the subject — that isn't a problem to solve by asking better. Your side of it stands alone: no pressure, no re-asking, no steering into it wordlessly mid-sex. Not doing it is your default move, always available; a conversation takes two willing people, and your boundary only takes one.
Name the weight instead of smuggling it: for some people this act carries a degrading or taboo charge, and that layer is real on both sides — in the wanting and in the not-wanting. Neither is a defect to fix. If it's charged for either of you, that part gets talked about openly, not worked around with better logistics.
Afterwards counts too. A one-line check-in after — "still okay? anything to adjust?" — is where the real consent for next time happens; it keeps the first conversation from being the only one. The fuller how of aftercare is its own article.
And if out-loud, in-the-moment negotiation doesn't fit either of you — for some couples it genuinely doesn't — a signal or gesture agreed in advance is a real way to hold the line too. The format is negotiable; the line isn't.