Because in reality it makes sense. You’re trying to get over that person and for whatever internalized homophobia. Yet in manga what’s so annoying about this specifically is the one who has the crush is allowed to have multiple partners, sleep around etc. yet the person who the crush is directed to remains celibate or single until the cowardly partner comes to terms with themselves. It ultimately “rewards” the character for having feelings despite doing usually absolutely nothing to show for it. It’s stupidly frustrating and makes the person who had the crush seem like an indulgent egomaniac who gets to have and eat their cake. It’s beyond unrealistic but it works in fiction somehow.
Sadly. It’s among my least favorite tropes and they do it a lot in romance plots. It’s always the popular guy and the shy girl he’s always secretly liked. It’s quite annoying and so unrealistic. And some the issue is always resolved with some big romantic gesture. I watched a movie the other day called “the thing about Harry” and even while the film involved gay male love interests even that followed this annoying trope. I really hate it.
Again that’s the point. That one character is allowed a sex life and often the other is manipulated or directly prevented from having one at the control and cost of the other. A ton and I mean a ton of BL plots follow this formula previously because it relies on heteronormative expectations of female characters in traditional romance narratives. It’s what they know so they just recycle it with two men instead. One character is allowed character development through their indiscretions which are nothing more than selfish indulgences. Sex is fun, but sex is not a necessity to everyone's. If sex is that important to you than that is a personal need. That’s fine. But another character shouldn’t be their reward for curving that vice. It’s objectifying. It’s sexist. It’s hypocritical. You can be celibate or promiscuous but you’re not owed anything for it. Nor should you be expectant to be rewarded for it at the cost of another’s autonomy. Which is what we have with one character intending to cheat on his gf and purposely cockblocking his best friend. It’s whatever in fiction but it perpetuates this idea that men continue to have about the sexuality of women more than anything and their assumed ownership of it Just here with two men instead. Fiction can perpetuate just as much as it can create harmful stereotypes, expectations, and exceptions.
The new WEBTOON Aquaman actually goes into detail and unpacks this trope more thoroughly specifically if you want to actually understand my point in a more entertaining way. More so because we’re actually given the POV from various characters in just how damaging this trope can play out if done realistically.
If I understand right, seme was already interested in the uke ? I know it's not easy to do (especially in LGBTQ's relationship) but why go out with a lot of people when you have already someone you've a crush on ?