I know for me personally the pain from the cheating wasn't in the sex with another person. I'm good with that.
The pain came from the lying about it, the saying they were in one place, when they were another. The loss of trust when all of a sudden you can't believe anything a person says because they aren't being honest.
I think if there hadn't been sex involved, yet I found out my SO was lying to me on a regular basis I would have been just as hurt. The pain for me was even worse because we had an open relationship and I had been blatantly clear from day 1 that I didn't expect monogamy, but I did expect honesty. I made that VERY clear. And I guess I felt like since I had required so little, to have that one little thing that I asked ran over was like being told I wasn't allowed to ask for anything. That my needs weren't important at all. That the other person would do whatever they wanted to get thier way without any regard to me.
Again, to me, the issue isn't about sex with someone else. It's about lying. And honesty is the one thing I am very serious about.