It was stupid of him to do it. There was no need. There’s no room for doubt despite it being early into things. But he had to know. One might argue that you should already be sure, but how can you be sure of anything. Heck, there are couples who are together a long time and doubt still finds a way to make room in one of their lives.
So he had to do it. It may not be right but at least he knows he was right all along.
Technically, things aren’t even “defined.” But how two people define what they have is between them, not societal norms. Too late now. He knows she’s here to stay. The night spent with the other one gave him all the answers he needed.