i'd say its not just you...have you talked to her about it? well that would probably hurt her feeling maybe suggest a activity for both of you like an afternoon walk around a park or swimming???

I'm personally not sexually attracted to obese people.

You both need to talk. You need to tell her what actually makes you excited, and I would recommend that you both start exercising together as well as watching what you eat.

The exercise will help increase the amount of endorphins that you release which will arouse you naturally. If you both exercise together, it will make you both excited, and you will have more sex.

Keep us posted!
Jim