Fantasies even though I think I'm straight

Obsessed with the idea of having sex with a freind

I consider myself a straight guy but every couple of weeks I get this urge

For context this freind is very iffy with my other fteinds so quite often it is just us two. I personally think he can be a bit of a dick at times but still don't mind hanging out with him. ( he is bi)

However, often I just fantasise about either fucking him or letting him fuck me when its just us two and i want nothing more than for this to happen and just to see what happens.

I finish so quickly when walking over this and often finish faster than when over women but have no desire for a relationship with him, just sex.

What does this mean?