Considering how much trouble Bethesda has with normal character development, I seriously doubt they could do a full blown romance right.
The whole "character development" thing is at bit at odds with the "open world" philosophy of Bethesda, to be fair. Bioware has an easier time because they're dealing with a much more contained plot. (And I'm not going to get into how I don't actually think Bethesda's games are really any less linear than any other...)
Bethesda's problem, as I see it, is not so much that they're somehow inherently poor at writing character development, but that they don't go to that next step in the first place. Each character you encounter (and this applies to videogames in general, really) is the same at the beginning of the game (apart from higher levels and skills) as they are at the end of the game. This is antithesis to basic storytelling - a key feature of any "good" story is that the character learn something through the course of the narrative (or are significant in that they fail to learn anything.)
They go so far as to create some interesting characters with a bit of backstory, but they're generally painted in very broad strokes. (Like I've said before: "Hi there, here's some interesting expositional stuff about me. Okay, well nice meeting you - I'm not going to have anything more to say throughout the rest of the game, so you can pretty much just ignore me now.") There are few characters that offer you any further insights, or respond to how events are playing out in the world. (Defuse Megaton's bomb, and the worshippers still talk about how terrible it would be if someone were to defuse it, etc.)
Bioware's games (which are notable as all your supporting cast has a slew of backstory to dig into and lessons to learn,) are set up specifically to take advantage of that. There's a noticeable pattern in all of their games, consisting of doing a mission or two, and then returning to a "hub" area where further layers of the characters are revealed. Rinse and repeat. That's a little harder to do with an "open world" game like Bethesda makes. (If your character can go through a city and talk to everyone - all the work of implementing some development in the characters are going to be wasted if the player never goes back to see what's new.)
With the addition of player housing, there's now potential for something like that. If I'm going back to Tennpenny Tower or Megaton on a regular basis to restock and drop off my loot, then there's the potential to draw me back into a progressing narrative within that area.
Back to the topic at hand - Bethesda may not be terribly experienced at character development, but the potential for a romantic interest of any sort would be one tool to use to realize that.
(And again, yes - it would have to be done well, or it would come off as amateurish and immature.)