I'm getting much better at resisting obviously selfish sex from him. I think what I struggle with in the senario I posted is that I feel he has a good argument. He is offering to give me pleasure as well which he is capable of doing and who turns down the opportunity to experience something nice?
Would you have a different response to that if you weren't thinking about yourself? I'm wondering how ingrained this has become. Do you believe that everyone should have sex with their partner, if their partner wants too, and they don't particularly but don't have a reason, or do you just not think that's an option for you?
Unfortunately you're not finding the answer that you want because it doesn't exist. If DP didn't want sex (in any capacity - from touching to oral or full sex), I wouldn't want to have it with him. I want him frequently, but only when he's in the mood. I would not want to talk him into it. He would not want to talk me into it. We'd never get to "I know I could make you like it if you give it a chance..." because that's coercive in itself, it's not really an offer for you - it's still for him. He wants to do something sexual. You do not. That should be the end - he shouldn't try and talk you round.
Who turns down the offer of something nice?
I do. Lots of people. I don't always go and see friends, because I don't want too. I don't always go for drinks, or out for food, or have my favourite meal. I don't have a Magnum every day. I don't wear my favourite outfit. I love all of those things, but I don't do them all the time - sometimes I turn them down because I'm not quite in the mood, even though I love them all. The same is true for sex. Sometimes you turn it down even if you know it'd probably be fine once it gets going, because you're just not feeling it.
I understand you're leaving him, and it's a work in a progress, and you don't want to anger him in the meantime - but it might prove critical both now and for your recovery once this is finally over that you can see that there isn't, and shouldn't be, an answer to this. You don't need a reason, you don't need to explain to him exactly why you don't want to.
You may well need to placate him, if you have to stay with him whilst you're making plans, and in that case, you might find it easier to be non confrontational. I can understand that, but be careful with excuses. He may well start turning them round and questioning them - how can you be tired, you haven't done much today? I'll help you relax, etc. He isn't respecting your right to say no and he's still coercing you, so you need to be prepared that your reasoning probably won't be good enough for him.
All the best, and I hope you're far away from him soon.