You and I are very similar. It feels completely natural to me that even though my wife is married to me, she has her sex with other, more superior, men. I get extreme arousal knowing that her pussy is accustomed to a cock that is much bigger than mine and that she is carrying another man's sperm inside of her, everyday, instead of mine.
I knew this about myself when I was about 19 yrs old. She and I were engaged and living together in college. I was sharing her with a college buddy of mine who not only had a huge cock, but had a stamina that I simply could not compete with. Our play started off as a threesome, but I could clearly see how much she preferred having sex with him instead of me. I just let nature takes its course. Before we knew it, I hadn't had penetrative sex with my fiancée for several weeks, even though her pussy was getting destroyed by his cock everyday, multiple times a day. My role had evolved into eating her pussy clean after he pulled out, and sucking his dick clean.
She finally brought it up. I told her that I completely understood why she preferred fucking him more than me and that I wanted that for her. I told her it was hard to explain, but it felt completely natural, to me, for him to be her primary sex partner even though she was my fiancée. It was just how my brain was wired. I told her that I loved her and that I wanted her to be my wife, but I knew that I was not very well endowed, nor good at sex, but I would always let her chose other men for sex, if she wanted.