When masturbating, Sometimes the only thing that gets me off is thinking about my man boning some random female. It used to make me feel bad to do this, afterward, but I'm starting to get used to it and enjoy the thought of it more (i don't think about the morals of it during). I have experimented with girls in my past, but I'm not sure if this fantasy is about that or an underlying self-esteem issue or something. I had a friend tell me years ago that they frequently imagined someone else in place of their self, but this person never dated and was rather self-conscious. I didn't use this method before I heard that, but I can't seem to help but do this every so often now. I don't mind thinking about threesomes, but for some reason I don't like thinking about him with just someone else. Usually in the fantasies he is cheating on me and sometimes I walk in on him and he disrespectfully keeps humping while I watch.

Try filming your own sexual act with him, but cover your face (wear a wig, or mask or something) This with some role playing, then you may be able to see (partly) how it may feel to you.
you might want to consider whether or not you want to have a more open relationship or just keep it as an arousing fantasy