I mean, it's not so much a GL thing as much as it's shoddy writing in rom(com)s where authors introduce a love triangle because they run out of ideas to postpone relationship progress with.
Not to defend lazy choices, but the introduction of a side couple or love rival is pretty much part and parcel for a standard romance beat. I personally blame the publishers who won't take chances on series that don't have a level of predictability (and by extension, profitability).
Staying zoomed in on the main couple without any external forces requires a good author who puts effort into character work, and that's honestly a big ask from your average mangaka - not to belittle the medium but using the visual aspect as a crutch for mediocre narration is a longstanding issue in manga, and the exceptions are often illustrator/author teams versus your standard solo mangaka.
If these folks wanted to, I wholeheartedly believe they could write a captivating slow burn romance without artificial plot extensions, or at the very least treat the side characters as more than a simple vehicle to push and develop the main couple. (In this series, Hinomori is literally just a reskinned Yaya with a pinch of tsundere added. There's nothing to her character beyond "competitive with mommy issues").
Sorry for the long ass comment, I just feel like this issue in the romance genre is so unnecessary, but until the way marketing and publishing changes it's going to be the norm until more "alternative" plotlines become consistently successful.