Millyay I think it sounds really unpleasant and I would find it really unacceptable.
I think you need to work out what you do find unpleasant or acceptable about it and talk about it with him.
If you like him groping your bum as a way of indicating he would like sex then fine, if you don't like it, you need to suggest some other ways he can signal an interest in sex.
I think referring to it as round one or two is awful. Firstly it sounds like a fight or boxing match! Secondly, it sound like it isn't finished. It's hanging over your head like some incomplete chore. That is how I would feel.
Anecdotally, I think it is quite common for women to have lower sex drives than men, especially after having kids, especially when the kids are young. So I think you are completely normal to feel this way.
You made it clear in your opening post that he goes quiet, which suggested that he does sulk, at least a bit, unless he instantly gets over the quietness!
"Well no sooner had I said that he suddenly went quiet, like I'd pissed on his cornflakes."
Yes, I agree with Eckhart ...
"There's a lot of worrying responses here suggesting that his behaviour is acceptable or that other people are experiencing the same and so they normalise it. It's OK to have different sex drives. It's not OK to try to manipulate your partner into sex by sulking."
I am so sorry for all the women on here have have experienced similar things.
AnneLovesGilbert " Plenty of people happily want sex when they’ve got a headache or backache."
How can you possibly know this? Is there a study? Your comments are quite unusual here. I think someone pestering for sex and sulking when they don't get it would definitely add to my mental load!