Responses
there's some details that's not the same as you described but it reminds me of that one scene in this manga http://www.mangago.me/read-manga/mujihi_na_karada/
Its this one! Thank you so much!!!!
I asked this before, but I got no answer. :( Can you guys please help me? I really want to re-read this:
The uke, I think, is in a car and watches the seme talking to his ex-girlfriend after meeting her at a party/goukon or something. The uke is insecure about his relationship with the seme (because the seme is/was straight) and expects the relationship to not last for very long. Which is why, even when it pains for him to see his seme talk to the ex-gf, he doesn't really kick that big of a fuss (but I think he asks the seme to break up with him later). But the seme is committed to the uke and loves him very much. Which is why when the girl was kinda hinting at the seme (or maybe she wasn't, I don't really remember), the seme told her to back off because he already has someone he is committed to.