Sex ultimatum?

I've known him for 10 years (he's friends with my sister) and we've been talking for 4 and a half years. When we first started talking it was all flirting and we knew we liked each other, we both wanted it to advance into a relationship and then my sister dropped the bombshell that she likes him and doesn't want me to do anything about the relationship (he's never been interested in her and for the last 3 months my sister has been in a relationship, still doesn't want me with him though) so very reluctantly, we haven't.

We still talk, flirt and now sext. I am completely in love with him, he's been there for me, he's that person you know you can talk to with absolutely no judgement and they'll talk to you about it but we have so much built up sexual and emotional tension, he suggested we have sex (men...) to help the sexual tension and we'd have to just deal with the emotional one... That suggestion was about 3ish years ago and still hasn't happened mainly because I'm a virgin and I wanted to make sure this was the right decision as we aren't together.

Obviously waiting 3 years is pretty long time which means the tension has built up more than ever. All this tension means that we keep having the same argument over and over again: let's just be friends > friends isn't working > let's have sex > just sex isn't enough (for me) > massive argument. He's clearly got very bored of this as I have, and in our just recent massive argument he gave me an ultimatum... We either stop talking completely or I have sex with him.

I am ready to have sex and he is one of my best friends but just kinda feels like I'm being forced. I can't stand the idea of losing him, I'm so confused.

I'm 20 and he's 28 if you're curious.