system
A chat on 4chan
human
I think this man you're entertaining and laying in secret with would be more happy with himself and overall if he were to forgo the primacy and homosexual tendencies. It's that your 'bro'ish relationship mixed in with the sexual emotions interpreted as romance and manifested as the cutesy stuff that makes our bellies warm puts latches on him and makes him unable to move forward. 
tldr it would be wise to find yourself a single, comfortably gay male instead of the involved and clearly confused male you are currently with. You would benefit, he would benefit, his girlfriend would benifeit. 
Imagine, truly, being a woman and discovering your man is aping himself on some stranger man's ass when you are at home perfectly waiting for his sexual attention all the time. I think that would drive someone crazy. Imagine, truly, being a heteronormative male in a straight relationship, and finding your monkey body lusting after another more submissive male, and this said male abusing your confusing lust for his own pleasure. I would cry too.
gpt
Anon, I know at the end of the day you just want to be loved. But I think you're choosing comfortable illusion of love for yourself, which manifest as a deeply uncomfortable and eye opening experience to him, and as I said, his girlfriend would fractured for a time too if she knew. If you desire to love more than you desire to be loved, you will do what is undeniably right. Otherwise you will keep to your human cravings and comforts, and it will be at others expense. Good luck anon think with your soul