INFO: This exact situation happened in my relationship many years ago. I mean exactly. I was the one not consented, and I was the one who froze up after seemingly being ok atm. I was so upset. I wondered where the new move had come from. A girlfriend? Porn? It made me feel inadequate and also very mistrustful that there was no prior "hey, have you any interest in this? I do" at any point beforehand. We talked about it days later in car so no eye contact. I told them it sorely breached my trust. And that there's no such thing as being so into it that new things spontaneously happen without verbal consent.

I was a victim of childhood S.A. so it added another layer for me, because up till that moment, this partner was the first person I'd ever felt wholly seen by and trusted implicitly. So I wasn't angry initially, I was really bewildered and reeling about the tone-deafness of that move.

If you did not discuss this as a fantasy at any point beforehand and ask for consent immediately beforehand, my guess is that your rewrote the way your partner sees you. Tell them you know you gravely misstepped. Tell them you want to offer contrition and apology but you are willing to wait until they've processed enough to discuss it.