Is it normal that i like to be naked around my best friend?

First: We're both not gay. Both of us believe to be straight men, maybe we're bi.

My friend and I hang out a lot together. We do sports, go to the cinema, have some trip, drink some coffee, go to festivals, cons, etc.

But one thing I found out about is that I truly enjoy (even to a sexual level) to be naked around him. Once we were having a shower together after swimming. Nobody else was there at the time. The room was filled with steam. Warm water trickling down our bodies. I was absolutely relaxed and a bit horny. I'm often just randomly getting horny, it's my libido driving me nuts. But when I normally need to get a stroke or see my girlfriend, standing next to him - even without seeing anything - I got a big solid boner. And I - damned - liked it. I didn't want to be fucked back then, I don't enjoy things stuffed up in my anus (I've experimented with my girlfriend a couple of times) but only being naked together with him. He first thought it was a bit awkward but pretty fast started to like it too so we kind of ended up masturbating together, even going that far as in me fucking his ass. This happened a lot of times.

I told my girlfriend about it after the first time I did it and she just giggled and told me that it was alright as far as he wasn't going to replace her and that she would have liked if I told her sooner. Didn't do so because I was afraid that she would give me a roundabout kick in the crotch and leave me.

Is it normal I like to be naked around my friend when I have a girlfriend at my side and I'm no homo?