I don't condone cheating.. but if some married woman is DTF I'm not in principle against it. That's her responsibility, her relationship is her business. If you slept with this woman, I'd judge her for cheating, and I'd judge you as successful at landing some P. (this isn't a gendered thing either, I'd say the same to a girl considering a fling with a married guy -- get that D! rock on with ya bad self! no shame in it)
Now -- If she has kids I'd stay away, I don't want to do things that could affect innocent children, but someone else being in a relationship and DTF is fair game IMO
I'm open to debate on this, of course.. If someone has a persuasive argument on how it's actually unethical to sleep with a married person, I'm listening