Physical activity is great for libido, self esteem, self image, erection quality, vascular health, and all kinds of other stuff. It cuts down on depression and anxiety, which can kill libido, and frankly it makes you look more fuckable. It's not just because of weight changes (loss or gain), but changes in skin tone and energy level as well.
After I started working out my sex life improved in absolutely every way. It starts with increased confidence in yourself and your body - which is very sexy. Fit people are also, frankly, better equipped to fuck well. I'm not talking about being an Olympian, either. You just have to be in decent shape to see improvement.