I always knew it referred to a second round of intercourse, made sloppy by the secretions of the first round.

I never thought (and maybe still don’t) that it required the sex partners to be different people for each round. I’m pretty sure I’ve seen it used in writing as if the sex partner was the same.

The reference I remember was some kind of comic take on dating and offering one’s date sloppy seconds as a kind of half-hearted show of interest and virility.

But it does add an extra dimension of crudeness to imply a second sex partner.