Beomgyu will never admit, even under the threat of death, that you were his first love.
He is well aware that one of the most pathetic things he can ever do is confess that he was in love with the crazy student council vice president he used to talk shit about back in high school, which is saying a lot, considering he does a lot of pathetic things all the time without giving a fuck.
He would also rather die than ever have to acknowledge that he sings about you. He doesn’t write about you, because he would rather die a slow, painful death than have to sing lyrics about you to anyone, much less with the other members; but he sings with you in mind.
You had always been awfully smug when he said anything about you, especially when he bad-mouthed you, and he would literally jump off a bridge if you ever found out that THE Choi Beomgyu, the class clown back in your high school, the K-pop idol who hundreds of MOAs are probably swooning over, the little shit in both education and TOMORROWXTOGETHER, sang songs with you in mind; no, sang love songs specifically with you in mind.
He knows, better than anyone or anything, that admitting any of the above will result in the non-existence of his dignity and self-worth.
And when Beomgyu accidentally runs into you while discreetly wandering around in public disguised, he immediately says adieu to his dignity and self-worth.