I stuck with Ashley. Many reasons: one being that I was already in a non existent relationship with one fictional character and I couldn't start chatting up another fictional character, it would make my make believe love life far too complicated.
Another reason being, non of the others really hit my magic 'impressed' button, the only one that came close was Jack and even though she got the silver ultimately Ashley still goes for gold. If I had to pick one from Mass Effect 2, it'd be Jack but since I had to option to stick with Ash, I did.
Finally I just liked Ashley and much like the OP couldn't bring myself to pick a different character when she seemed like the perfect match for my vision of Shepard. Sure there was tension now but hey, it makes the trilogy more interesting if the main hero has only one woman.
Plus it makes the hero himself more sympathetic if just before the mission he looks at the picture of his star crossed lover, rather then just go and get laid. It gave the scene more emotion that way, I felt anyway.
So yeah, to summarize, stuck with Ashley through the whole game and will probably continue to do so, but if I were going to pick another it would be Jack.
But still, Ashley for the win.