Personally I try to avoid being involved with people who try to emotionally blackmail me into bolstering their self-esteem, but that's not a lot of help once you're already involved with them.

In that situation I'd probably try and turn it into a strategic joke by saying something like "Eugh, no...look at those horrible shapely legs and disgusting pert breasts; yuck, no way," or something to that effect. Bonus points for picking characteristics where your girlfriend has the edge.

Use this at your own risk. I take no responsibility for its efficacy.