Firstly if his "demands" are in any way forceful or threatening then this is abuse and you should seek help for this as mentioned above.
It's also quite hard to offer advice as you don't provide any details about your relationship besides this problem.
On the surface: your partner obviously isn't happy with sex in the relationship. The groping and demanding sounds like a clumsy and lazy way of trying to initiate, and he probably feels like you wearing clothes to bed makes sex less likely.
This problem seems unlikely in the context of an otherwise healthy and happy 12-year relationship so I would guess that this is most likely not the only problem. Arguing? Disrespect?
My suggestion is to really think about whether you want to stay in this relationship.
If you're not having sex, then unless you tell him why not, your partner won't suddenly stop wanting it just because you keep saying no or start wear pyjamas to bed.
Instead of him constantly raising it, you need to own the conversation. And crucially you need to be honest and frank with him. Otherwise he will just carry on pestering you.
If it's simply the case that you'd prefer he initiated in other ways, then tell him that, ideally with some suggestions of what they might be.
But if you don't fancy him any more, or he's given you "the ick" and you don't want sex with him, and you just wish he'd leave you alone, that's fine, but you need to tell him that. Spell it out so he is in no doubt about it. And remind him of it if he forgets.
But bear in mind, saying "I no longer want to have sex with you, please cease trying to initiate" may well end the relationship.