I think there is information we are missing here, OP.
If you haven't wanted sex for five years, but have it anyway, once a month, it's a problem. I expect your DP knows that you have sex under sufferance and at some point years ago, found that hurtful and rejecting. Now, I suspect he resents you for it.
I think he has got so used to this situation that in fact he might have detached from you emotionally and physically and finds that actually, he wants sex with you less than he once did. So I think he gropes and mauls you at inappropriate times knowing full well that this won't lead to sex, but because it's you saying no (again) he can blame you. Because he has detached from you emotionally, he isn't bothering with romantic and loving seduction techniques, because sex now is just about orgasming, not making love. He still wants sex, but not necessarily with you.
With hindsight, he should have sat down with you years ago and told you how hurt he was at being rejected and tried to reach some understanding with you about why this happened and what compromises you could reach.
You perhaps shouldn't have bargained with yourself that it was normal and understandable not to want sex with someone you're meant to love. I strongly disagree with posters who are bargaining that this is normal and acceptable in long-term relationships, even those with small children. Good sex is like a glue in relationships and if you ignore its benefits, other aspects of the relationship start to unravel.
So, just like he should have talked to you about this long ago, I think you should have talked to him too about your lack of desire.
Instead I think what's happening here is a script you've got locked into; a dance that you are perpetuating.
What will happen if you both ignore it and keep re-enacting this dance? I expect he will develop (or increase) a porn habit, which will further serve to dehumanise sex for him. In fact there might be yet another link there between a porn habit and his inappropriate behaviour. Next, I suspect he will have an affair. He might not go looking for one, but he will not turn down an opportunity to be with a woman who desires him intensely.
Assuming you haven't gone off sex completely yourself and it is just sex with him, then I think you are equally vulnerable to an affair.
A really good, no holds barred, honest conversation could stop the rot here. If he's not good at talking about things, that might prove difficult on your own and so counselling might be the catalyst you and he need to start communicating with eachother properly.
I think your situation could be summed up by resentments on both sides, poor communication and very low emotional intimacy.