Sorry guys, something might be messed up along the way in there. On the original play through where I had multiple xxx events with Sgt David, I was spending all my time with Thomas but also had the David interactions on the side. I think it was after the date into town with Thomas that I suddenly switched to the David storyline, but no idea why...
I'd just say try to play like you're going for Thomas but then make sure you're seeing David when you can and play around with the Thomas storyline.