Soul never thought of Maka as more than just his meister. Sure they're friends and all but he never use to think of her as his 'type of girl'. I mean for one she was flat chested, two all she ever did for fun was read stupid books (not to mention hit him with one whenever he made a sly comment) and three no matter how hard Soul tries to be nice (and I mean really tries) Maka always misinterprets it for something else and Soul ends up face first, on the ground with a sore skull
All in all; Maka was a pain in the ass.

By now I'm guessing you're wondering why Soul would even bother to stay with someone that pissed him off so much that he would rather listen to Excalibur sing for eternity until his ears stopped bleeding (yeah she was that annoying). I'll tell you why.

It's because no matter how annoying or how immature the young meister would get, one thing would never change for Soul; and that was his love and devotion to her.
So what if she was flat chested and a complete book worm, it didn't matter to Soul; after all that is his type of girl.