They're both crazy with each other, alright. Seme knew everything; his last line "Its nice to be alone, right?" I think Seme just wanted to be alone with Uke with no audience, HENCE that planned chance encounter with Kaoru(uke's big bro) so that stalker would divert his attention to a new target.
Basically Threesome. God, so kinky (´ڡ`@) Sukuna is such a simp.