Ich habe gesehen, wie sich zwei Kerle auf dem Flur geküsst haben.

Is 'Kerle" rude in ordinary speech? The little vernacular German I’ve listen to used “Leute.”

In old German “Kerl” means “free man” and it is close to the names “Karl” and “Charles”.
Today it can be rude, but it can also be used jokingly.
In this particular sentence, the speaker might want to say that the two persons kissing were not only simply male but also masculine looking.