Again, the only one being immoral here is the lady, assuming her husband doesn't know about it and would be against it. You're the one placing your morality on other people, and I guess that's my point. As I said, I probably (hasn't happened so who knows) wouldn't have sex with a married woman, especially if I knew her husband would disapprove. But tell us more about your morality. After my first divorce, my ex-wife remarried, and we were still great friends. She came to visit once and gave me the best BJ I've ever had. Was that immoral?
Now, the new husband had wanted to see her give another man a BJ (weird but I try not to judge too much), and she was down with it but wanted to do it with someone she was comfortable with, hence me. In this case, I don't find it the slightest bit immoral as all parties were consenting (I even spoke to the guy afterwards about it). Lucky for me, the video camera didn't work so it was kind of a bonus. But, not knowing these things, one would consider it immoral. Now, tell us, how far does a man need to go before having sex to have a clear conscious? Sex by itself is not an immoral act. The woman was being immoral if she broke her vows/arrangement. Onoes did no such thing.
As an aside I would recommend, for practical purposes, that Onoes tries to stay away from married women in general, though, cause some guy not as even-headed as we is going to find him one day.