You can make all sorts of romantic decisions across the Mass Effect games, each with their own distinct narrative arcs. An unexpected one actually happens based on the choices you don't make, and it was so surprising, it caught a developer completely off guard.
In Mass Effect 3, if you don't lay it on hot and heavy with either Tali or Garrus, they will hook up. It's a nice bit of narrative flavoring that makes your crew feel more alive and autonomous; if you've a personal life on board the Normandy, so do they, and even your rejections have consequences.
