You have 4 choices.
Smile and go through the pity sex until your SO loses interest. Which is a gamble cause hey it could be a month or it could be years and years.
Talk to him and tell him that you no longer want to have sex with him. You can tell him it was all the years of rejection or you could just say you dont feel like having sex with him anymore. Of course the drawbacks there is he could request or demand therapy and want to work it out and if you dont respond in kind he may actually leave you.
Tell him you no longer see him as a sexual partner and then lay out a plan on why you shouldnt divorce and how/why this partnership should continue as a open relationship. Hope he doesnt freak out and ask for a divorce or just becomes bitter and angry thus destroying the only part of the relationship that works. Your friendship.
You leave him. Which you said you wont do but it is still an option.