Is it unusual for a straight male to fantasize about another male?

I've got a bit of an embarrassing situation.

As far as I know I'm a straight 21 year old male. All of my sexual encounters have been with females. The thought of doing anything sexual with another guy doesn't do anything for me and I'm genuinely not attracted to the idea of being with men, although the penis is a fascinating object. I am aroused by the opposite sex and the only porn I can get off to is M/F or girl on girl(I've tried gay porn and it wasn't doing it).

Here's the weird part. I know male anal stimulation(or prostate rather) is perfectly normal and I've experimented with it when masturbating to where I'm fairly good at it. Recently when I really get into the zone, I can get off that way when fantasizing about my male friend. Just imagining him ravaging me and having me being on the receiving end of him makes me crazy. I can get off to fantasizing about doing just about everything sexual with him, but it's only him.

I didn't think of him in a sexual light and I still don't when we're hanging out or whatever unless I'm really horny. I've literally never thought of another male in this way and it's a bit puzzling.

Does anyone have a similar experience? What is this phenomenon? Thanks in advance.

TL;DR - I can get off to anal stimulation while fantasizing about my male friend doing dirty things to me and it's confusing.