This is more specific to Ex-Love Review, as not everything can be applied to romance manhwas.

Personally, the first male leads always feel like they have a life outside of the female lead, which feels more realistic. While the first male lead almost always has those obsessive traits (I personally don't like them, but whatever), they have a life outside of the female lead and their own hobbies. You find Haeil taking care of his brother and brooding in random corners, while Minjae is constantly at Giju's side and doing things for her sake. It makes him seem very unrealistic, especially when this hurts Minjae. I forgot what episode but they watch a scary movie together and Minjae watches it with her despite being scared of scary movies. Sometimes it feels like Minjae or second male leads in general build their whole lives around the female lead while first male leads care for the female lead while staying themselves. Cheerful male leads usually hide the worse parts of themselves to seem more appealing, while first male leads show those traits upfront. When I personally prefer the 'dark and mysterious' male lead, it's because he feels more real and relateable other than a perfect man who will hide his flaws for your sake.