In the beginning, a one-night stand is just that. A hook-up. But what happens when that "one-night stand" becomes a regular event? Can still fit definition of hook-up at this point. Slowly, emotions get tangled up. You now have a situation. Particularly if the other person you are in the situation with is living with his sort-of/common-law/fake wife and has two children from previous relationships. Situations get messy, especially when you fall in love with this individual. You're spending more and more time together, it's been a few months, and there are real emotions at stake. Your situation is no longer a situation and not quite a relationship. It's a situlationship. Hopefully, to eventually progress to a relationship.
Yeah, it's all kinds of wonky, but it's our situlationship and nobody else's.