I don't think it's wrong.
First, if I sleep with a woman who willingly cheats on her partner, and she believes cheating is wrong, that relationship is more or less doomed anyway. My NOT sleeping with her won't change that.
Second, it's her life, her relationship, her values, her responsibility. She wants to sleep with me and I say no because she has a partner, I assume a responsibility that isn't mine, and I belittle her in the process. I may say no for other reasons, but not for the single fact she has a partner.
Third, she may see cheating as perfectly normal - cheating is aligned with her set of values, even though society sees it as bad.
For me it's theory BTW - so far I seem to attract only single women who want a monogamous relationship
.