What's wrong with that? They weren't married.
She probably stiffed him on the babysitting money.
I see your point: I often say that there's no such thing as "cheating" if you're not married. The whole point of not being married is to be able to pursue other romantic options.
However, this logic does not encompass the emotions of the uncommitted but immature. I saw this even among my own friends in the 1980s: a single date created a sense of ownership, for many.