Unless one has a medical condition, no one should ever diet. People need to make lifestyle changes. Eat 6 small meals a day, don't eat processed food, educate yourself with nutrition labels as in what should be in your food and what should not be. Exercise everyday, even if you just take steps instead of elevator. It's better than nothing.

Regarding being skinny, I struggle to keep weight on so I have to follow special diet. I used to think that because I'm skinny I was healthy. I learnt that is not the case. It is what you put in your body and exercising that counts.

And yes, sex burns a lot of calories.