I was thinking more in the sense of "doing something that will lead to breaking-up / divorce" and "acting as if you're already single when you're not".
I think the joke is they want to MAKE themselves single, but also that they ACT like they are single. But yes I'd bet the average cheater is in denial about both sides of this shower thought.
Cheaters tend to be controlling in their relationships. They crave that control, so they don't actually want to be single. They just want all of the power in the relationship.