Because even in real life, most people don't have their first time with the love of their life (if that even exists), or even with someone they actually are in love with, period. It happens, it's realistic, it's good.
Chu Sam is pure heart and honest that's why in reality many people always play with them yet love in deep with the playboys instead. And when those playboys cheat on them, they start to return back and said "sorry, I'm just secondhand". That's stupid!
Your logic has no logic whatsoever.
The story looks to good but why should it be that fu*king damn man who he slept with first? Why not it be Chun Sam?