Is he a rapist? Legally probably not.
Is he having sex with you and hand on heart thinks you actually want it? Probably not. I bet he's not actually thinking about that though, because if he was, he'd not keep on at you.
OP if you don't want sex, talk to him about it out of bed, tell him you don't like this pestering on Sunday mornings and then next time he does it absolutely put your foot down. He is not entitled to your body.
There is a violation of respect (at least) going on here. With regards the cake analogy above, it depends exactly how the asking was done, if it was a simple question, that's one thing, if the person took a finger of icing and was trying to put it against my lips and i was pushing them away, or turning my head away or trying to ignore them it would be different. If I felt I couldn't get angry with them because they'd be a PIA sulking for ages so just opened my mouth and ate the damn thing to get them to leave me alone, I'd have hardly enthusiastically consented.
So why does your DH want to have sex with you when you don't really want to? How can that be a turn on? And is he wanting to make love with you, or he's woken up horny and you're there? Sorry because it's not nice to think about, but it relates to respect. And while rape is a heavily charged legally, defined word, there are other things that happen that wouldn't make it to court, but also aren't ok.