The problem with labeling Stewart a slut for having an affair with a married man is that you're conflating "having sex" with "doing something morally shady".
Slut epithet and gender aside, the fact that one of the partners was married is salient if we're talking about morals, ie, it's a bit more than merely having sex.
That was my point--the bad part isn't that Stewart had a sex, the bad part is that Stewart (and the guy, I forget his name) made a bad decision. The bad decision involved sex, but it also involved lying and oathbreaking. The sex is how the oathbreaking happened, but there are other ways of breaking marriage vows that don't have to do with sex. Making it about the sex (and specifically about the sex that Stewart was having) is engaging in slut shaming, which is the idea that women who have sex are doing something bad even if it's enthusiastically consensual sex between all parties involved.