Thanks LucyG, I've quoted your paragraph above because I think this exactly what he is fearing. He said he doesn't think we are having an affair but that eventually we will. He thinks like you say that at the core all men want to sleep with women they are friends with.

I can't know what is really in f's head, he shows no outward signs of attraction, he's straight and he's told me about his crushes and I've noticed him checking other women out when we've been places together. At the same time his closest friend is a lesbian so he is very capable of relating to women as friends in non-sexual way.

Regardless of the reality of f's intentions (or not), I do think you are right - he sees f as competition. I have tried reassurance that I know the difference between friendship and the love that we have together. Not sure what else I can do short of losing one of the few trusted friends I have.