He's got evidence that she had worked at the club as a hostess. It's incriminating enough of a thing to put strain on the marriage; he doesn't need any actual porn of her for her husband to presume/suspect this is what she was doing there, no matter how much she claims otherwise. Pretty much the same way you're making this presumption here.
Also, you seriously think Marcus would be happy with the idea that his wife is cozying up with other men for money at a shady club where things can easily escalate into fucking, that he wouldn't mind her getting back to that work?
It's not about being nice. Getting to share your lifestyle with someone, especially if it's something you can't talk about in the open, can be very much self-serving. As can be playing mentor. Also, "this is this and that is that". Fucking your students doesn't mean you can't be a charming individual with people your own age.
Of course, it's entirely possible it is me being wrong here, but the way we saw those two (briefly) interact didn't really make it seem like there was some dom/sub thing going between them.
Python:
et "Marcus?!?"
n "He was standing on the sidewalk, speaking animatedly with a woman... no, laughing. Emily’s stomach tightened when she recognized the other figure."
et "...Mrs. Harris?!?"
n "They seemed familiar. Too familiar. Marcus leaned closer to her as she touched his arm, smiling."
It feels more like peers flirting, than anything else. Given they're of similar age, they could be old friends from college or whatever. But guess we'll see.