Any manga where the 2 leads like each other/are dating/married/etc, but then at some point in the story, someone from one of the lead's past appears that causes conflict in the relationship. A love triangle, but that other person is connected to one of the leads, preferably the female lead.

preferably it happens to female lead

They end up/stay together in the end. The first love/ex is only there to cause conflict.

Examples: Yume de Aetera

Fuguno and Nagisa are basically in love with each other, but then Minato, Nagisa's first love, appears.