&gt;do couples find each other less physically attractive over time when the newness wears off?

Wanting sex less because you've fallen into an unexciting pattern, or started taking the other person for granted, or just can't prioritize it over other stuff, possibly.  Being *so* turned off that the *only* way you can get in the mood is by imagining other people?  That's a serious problem.

Frankly, you may think this girl is nice, but it doesn't really sound like you respect or appreciate her as a person or a partner deep down.  Let her go find someone who does, and look for someone who engages you on both an intellectual and a physical level.