Uke's insecurities stem from the fact that time and time again the men he had experiences with were either curious or had dating experience with women. His insecurity didn't just appear overnight, and yeah, it may be stereotypical but when you've been chucked out like you're worth nothing and just a "substitute for the real thing" some people would start to believe it to be the case. And the seme isn't really a different case, one of their convos were that seme didn't really know he was attracted to anyone until the uke. To the uke, that seemed like enough reason for doubt to fester, I mean, he might find someone else. And just a personal theory, its maybe because of that very uncertainty, and the lack of exploration on the seme's side that gave birth go genuine fear coupled with ukes previous exes experiences. ANYWAY, not saying that these excuse anything, I'm merely providing a perspective as to WHY he'd think that way. The queer community is very divided, as some who experience predominantly same sex attraction fear those who experience both sex attraction in their partners in fear of being abandoned. As this phenomenon is very common in spaces where queerness isn't really accepted, so often those who go back to what is seen as a socially accepted norm such as a heterosexual relationship are often driven to do so by fear of social rejection. Heck, a lot of queer people don't advise getting in a serious relationship with sexually curious and or experimenting people because it might lead to more bad than good.
Uke's insecurities stem from the fact that time and time again the men he had experiences with were either curious or had dating experience with women. His insecurity didn't just appear overnight, and yeah, it may be stereotypical but when you've been chucked out like you're worth nothing and just a "substitute for the real thing" some people would start to believe it to be the case. And the seme isn't really a different case, one of their convos were that seme didn't really know he was attracted to anyone until the uke. To the uke, that seemed like enough reason for doubt to fester, I mean, he might find someone else. And just a personal theory, its maybe because of that very uncertainty, and the lack of exploration on the seme's side that gave birth go genuine fear coupled with ukes previous exes experiences. ANYWAY, not saying that these excuse anything, I'm merely providing a perspective as to WHY he'd think that way. The queer community is very divided, as some who experience predominantly same sex attraction fear those who experience both sex attraction in their partners in fear of being abandoned. As this phenomenon is very common in spaces where queerness isn't really accepted, so often those who go back to what is seen as a socially accepted norm such as a heterosexual relationship are often driven to do so by fear of social rejection. Heck, a lot of queer people don't advise getting in a serious relationship with sexually curious and or experimenting people because it might lead to more bad than good.