NSFW - Open Relationships?

Hi All, I just wanted some communal advice on this dilemma I have been having. As a little bit of background, I have been dating the most amazing guy for almost a year now- I could not picture my life without him. I am 22 (he is 20), and for him, this is his first long-term relationship. I am definitely not someone that values sex above anything else in a relationship, but I do enjoy (REALLY enjoy) sex. The only issue is that sex between him and I doesn't really work out. I am more of a top, and he is more of a bottom (Just for the sake of giving 'roles', but are both somewhat versatile), but he can't bottom for me... it is an issue with size and comfort. He doesn't particularly like to top often and I don't particularly like to bottom often, and this has affected our sex life majorly- we haven't had sex since we first started dating almost a year ago. Yes, we are intimate with each other, but the physical act gets old and tired often and we have both recognized this and for a good couple months we weren't even intimate.I have been wanting to talk with him about opening our relationship up to platonic sex with other people. Sex is not something I am willing to lose what I have with him to, but at the same time, we recently had a conversation where he told me I need to be prepared if we rarely have anal sex, and that for me was a huge letdown for me. Because of this I have been pondering the thought of what it would be like to have a friend with benefits or a purely sexual partner outside of myself and my boyfriend just for the sexual pleasure, and maintaining the intimacy and connection between my boyfriend and I. He would love to bottom given I had a member he was more comfortable with, and at the same time I like it rough (sorry TMI!) and would like to experience that and experiment with that side of my sexuality. Has anyone had experience in a monogamous relationship and testing the waters of having outside purely sexual partners/friends?