As far as cheating goes it is probably among the lowest forms
I mean say hooking up with someone on a one off, say drunk in a nightclub, at a friends stag/hen do is probably the least bad. Tis a one off stupid mistake, might cause the relationship to end, might just cause lots of humble pie to be eaten...
Falling for someone else at work, over time, a friendship becoming something a bit more... That probably is getting a bit worse, you didn't mean it to happen and but it isn't a random one off situation.
But actually going on a website and actively trying to find someone to cheat with - that is low even for a cheater - I'd wager that a lot of the people who cheat have either hooked up with someone on a drunken night out or fallen for someone over time but in either case weren't actively trying to cheat. Signing up to a website to intentionally look to do it is especially scummy.