I think monster hearts made some splashes because it has rules about sex. Never played it but it was one of the bigger pbta games.
My pandemic DND game had several rom com subplots, but everything was PG rated. The players got the tavern keeper shy man to finally ask out the Scary Strong blacksmith lady. Later, the self described “gay gay homosexual gay” player character had a whole Thing with a NB arcanist they met at a party. Lots of awkward uncertainty and innuendo. Very good time. Another player declined to pursue a romance subplot with an interested npc. All of that was fine. We didn’t use much systems for it because DND is very bad at that.
A game of vampire I ran pre pandemic had a PC exploring their relationship with a mortal woman. Their actions got the woman shot by their enemies , which caused the woman to Awaken as a mage. That story had legs. Minimal mechanics for it, but there weren’t a lot of conflicts to roll for with the relationships anyway. Sexual content was mostly off screen.