In "sayonara alpha", it was omegaverse. In this "universe" there are destined pair (Alpha/Omega) ... or soulmate if you like. Sadly, in this situation, the 2 of them meet way too early (they even say it in the manga).
When a destined pair" meet, the omega's body automatically goes into heat even if it's not the usual time. It's a biological reaction for which they don't have control. Besides, the omega (the older) does everything not to give in despite the physiological reactions of his body. He even takes pills to stop these reactions and even endanger his life.
However, it is ineffective and to solve the problem, the alpha moves elsewhere. The omega waits for him years until he returns (after finishing high school) to live with his lover.
when i first read this a few years ago i totally loved it, but now i just can't take this seriously i mean he had sexual desires for an elementary schooler. couldn't yuuji wait a bit longer till middle school i guess. i dont even know if i want to laugh or be appalled (/TДT)/