I wouldn't worry about it.
How to put this....
okay here goes..
A lot of ppl have fantasies. It's something I feel you generally would keep to yourself though. Like lets say you had a blonde hair gf and say you have fantasies about a brunette, you're sure as hell not going to tell your gf about this are you. Do you get my point.
I have a bf and i too have an imagination but he will never know most the things i think about. it's personal..
There's nothing wrong with it though..
You prob think of your friends mother because you see her a lot & find her attractive. no harm done. don't be embarressed but maybe keep try & keep it to yourself.

I bet your buddy thinks about your mom too - it's all cool. Maybe you guys could work something out?

dude, i masturbate about you and his mom all the time... it's cool

Hey steven89
When you're a boy in your teens and not sexually active addiction to masturbation and porn is a lot like addiction to air. It's just part of your existence. If you find yourself obsessing on one fantasy you may try forcing yourself to masturbate to different porn. Continuing to indulge an obsession just throws fuel on the fire. I think you understand this isn't a healthy or realistic fantasy. Time and diversity will get you over it very normally.