Of course, male/female friendships exist. When I think about it, though, my male friends are all people I knew in another capacity first: classmate, flatmate, colleague, people I've worked with over time on political/social causes. I can't really imagine meeting a man in the wild and unproblematically becoming besties. I also think the sexual tension issue is variable: some people are attracted to quite a lot of people and others very few. It wouldn't bother me if a friend were sexually attracted to me and loads of other people, but if knew he was ONLY attracted to me it would be awkward even if he never acted on it.
His partner has told him he now needs to cut all ties with me. This part makes no sense to me. (I'm assuming you turned him down and he's staying with her but she found out what he said to you?) If my partner told someone else he wanted to split up with me to be with them, he would no longer be my partner. Why would I still be interested? That has nothing to do with whether the other person reciprocated or not. (I'd also think he was an arse for hedging his bets and making his move before breaking up with me.)
Oh, struck out with Magenta, eh? Guess you're stuck with me until someone else comes along. And by the way, stop calling her! Yeah, can't see it.
Im now blocked. I've lost my best friend overnight and its a huge shock to the system. I'm sorry this happened. It is shocking and painful, especially when it's a long friendship.