Another game [series] you may want to check out (for reference purposes) is FABLE 2. You could woo almost any character.
This is hardly a good option for research into in-game romance, except maybe as an example of how little it takes to include a "romance" option on the box. All the woo-able NPCs were cardboard cutout characters: their "personality" ammounted to three randomized stats determining things like what gifts would impress them and if they'd gladly sleep with a complete stranger. No seriously, at one point my wife and I ran our hero around the pirate town propositioning every NPC we could find to see how big of a group we could take back to the inn. I think we rounded up 60% or more, roughly everyone with a "promiscuous" stat. What can I say, it was a seedy place.
Hilarious, but hardly something fans of dating/romance sim interactions would consider part of that genre.
Any storyline to a particular player-npc romance would have to be invented by the player: there wasn't so much "romance" as a collection of facts and stats. Yes, we are married now. I place the collected spouse NPC in one of my collected purchased homes. I choose to add collected offspring to the inventory by selecting the "have children" lever. But at no point did you have any kind of interaction with an assembled personality.