It's really hard for me to understand feeling this way. Every partner I've had has been different. I guess being bi and a switch, I've had more variety in sexual partners than most, but even so, no two people feel, taste or move the same way. Everyone has their own surprising qualities and habits. You say "a penis is a penis," I say "how long? How wide? how hard does it get? What color? Circumsized or uncircumsized? Any piercings? What positions does he like? What kind of noises would he make?" And yeah, when I see an attractive man I can't help wondering about those things, and I would wonder even if I were in a monogamous relationship. I think that's natural for an awful lot of people, even if it isn't for you.
Personally, I'm poly and very happy that way. But even if I were in a monogamous relationship, it would be hard for me to see cheating the way many people seem to see it, as the worst thing you can to do someone. It just doesn't seem like that big of a deal to me compared to other things, like "financial cheating" -- i.e., lying about where you're spending money, having a separate bank account separate from your spouse, etc. Or the kind of everyday verbal abuse that I hear from a lot of couples who've been together for a while, like "you're so stupid" or "shut up, you bitch" or whatever. Those are things I would leave someone for. Sexual cheating wouldn't bother me as much, since, as long as everyone is safe and avoids diseases, it wouldn't really hurt me.
My sister-in-law is very Christian, and is always after my husband and me to "forgive" his father, who has always been abusive and called my husband a liar in a court of law when he was a child. And yet, when one of the sister-in-law's friends' wives cheated on him (the friend and his wife have two kids, btw), she told him not to forgive her, and to leave her right away. I still have no idea what kind of mindset makes a cheating wife less forgivable than an abusive father. It's honestly beyond my comprehension.