We understand the seme is actually in love with the uke (tsundere) while the uke keeps on playing around and finds a new boyfriend. 1y later they meet again, the uke is still with the boyfriend (and some other sex-friends maybe?) while the seme is going out seriously with a girl, which troubles the uke.
==This type of manga is not for me. In my opinion there is no love from the uke, or at least not a sincere one. Just a feeling of superiority, possessing something you know it will always remain yours. But once you see it moving away from you and getting into a serious relationship, you tend to come back to this 'belonging'.
Actually, I think it would have been better to add at least one or two more chapters to see if the uke would eventually stop seeing others to stay with the seme, not bearing the seme to be with someone else or the seme dumping the uke for good and making him realize what he is losing, or any other scenario, but make a proper ending.
==I hate this kind of uke badly, but the seme is also pissing me the hell off to accept to destroy a potential happiness for a selfish horny and unfaithful uke. But well, it is his free decision. :)
Michi Dzure