People have lost thier SO's because they opened up, and their SO liked the other person more, because of sexual skill or whatever else. It's a risk of any relationship, and there is a greater chance of our happening in open relationships simply because your partner is likely sleeping with other people.
That being said, did you ever consider leaving your SO for your new date you had sex with? Did her vagina feel better? Tighter? Did she have larger or prettier breasts? If any of these things were true, did they make you forget your love for your SO? Did it ruin sex with your SO? I doubt it...
Obviously it's different for men and women, physiologically and societally. Men's sexual organs basically have a decent time during PIV, physically at least, regardless of person they are having sex with. PIV for women can be more complicated, not all PIV sex is pleasurable, or leads to orgasm. Even worse, men AND women are told that every PIV encounter should end with an orgasm, if not a mutual orgasm, which is a ridiculous standard that few men live up to.
Regardless of how ridiculous the standard, it weighs on men, and makes almost all men insecure about sexual prowess. This is further exacerbated by the penis size fiasco.
Anecdotally, I am an individual with a large penis. I was left by an SO I loved very much because she enjoyed sex with a man with a smaller, uncircumcised penis that felt better. We both cried a lot. I called her callous and sex-obsessed, and couldn't understand how she threw away all we had for sex, especially since we had a great sex life, and I made her cum all the time from oral (like twice a day at least), and occasionally from PIV. I thought that I was a fool for being open and sharing her, and that I lost something I could never get back.
I was wrong. We weren't meant to be together, partially because she was immature but mainly because we weren't as sexually compatible as I thought. Maybe ignorance would have been bliss, or maybe she would have resented me.
I'm mono now with sobering