To add a depth to the game that makes you want to roll a character just to explore the depth you can't get if you play a certain way.
Is that fun depth though? Sounds like needless restriction tbh. Characters having specific sexualities can be fine, especially if it's somehow plot relevant. But I doubt it's going to be a relevant story point in this game and it is, at the end of the day, a role playing fantasy game. Locking the romance for every character to specific builds feels like it would be frustrating padding than depth. If I've made a character a specific way and ended up feeling like they've had a certain rapport it's not going to add to my fun finding that there are no romance options. Sure, it would be realistic, but we're not playing D&D to have a realistic time.