Dating someone who is in a closed relationship is just beyond shitty. Hell, even pursuing someone in a closed relationship can create serious strife in other people's lives and is a pretty fucked up thing to do.
It is one thing in our teens/early 20s, but once someone is in their 30s, 40s, 50s+ and is more likely to have dealt with real relationship shit, like kids or dealing with serious financial crisis, mental health crisis, physical health crisis, etc., all while trying to keep food on the table and plan for the future that is some fucked up shit.
If people want to physical try something new, swinging is a thing and isn't cheating, open relationships are fine (even if they're a sign that the relationship is probably over anyway), and breaking up is the adult thing to do prior to any shit like this. Cheating and pursuing those in closed relationships is a good way to fuck up a lot of people's lives.
https://www.youtube.com/watch?v=VvYIpa1Ulvw