Seme-chan didn't sleep with other men to get the uke jealous. He was rejected at that time, they have NOTHING and try to go partying and sleeping around. The uke knows that and it is why he isn't mad with him, the seme is just happy bc:
1. they are talking
2. the uke cares about him, get jealous about "his past" without being a creepy boyfriend.
Uke-chan is dissapointed you are unfair whith his seme ヽ(`Д´)ノ
But... doesn't he live in his sister's departament? (╯°Д °)╯╧╧