I don't ever see Shepard settling down. It's kinda like how Anderson never remarried, even though him and some other scientist chick hit it off real nice in the first book. But with Shepard, it's more of a, "I've been serving on star ships since I was 18, I'll never REALLY be done defending the galaxy" thing. I think, the romances should be a touch more complex(like Jack's romance, but not a one time decision), and if you get to the point that you both consider and have expressed interest in getting married/attached, a nice ambiguous scene at the end where Shep is proposing would be awesome. Then they just have a nice semi-permanent spot in your crew.
But yeah, Shepard is never going to just "settle down" somewhere with an LI. Shepard is a career soldier, a commander to the end. They might try to shove him/her into an office in the end, but Shepard would just blast a hole in a space station wall/bulkhead and escape in the Normandy VIII (a stealth super dreadnought XXL) and fly around the galaxy saving it from all sorts of threats no one else but him/her and the crew believe exists, because it doesn't when they come around to check it out.