Yaar, Jaya, don't really know if sex increases it or not, but one thing our bachelors groups keep discussing all those beautiful babes, who got married and their figures started bulging. May be, it's related to this activity.
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Weight increases when you are happy. You are happy when your health is fine. You feel happiness in everything you do. So, good health is key to happiness. A good body when do sex, it leads to happiness and excitement. When you are happy good hormones secrets in your body. Secondly, when you do sex you get sound sleep. Fats grows in our body when you are in sleeping state. That is the reason why you gain weight when you do healthy sex.