Hello. Welcome to hcm.
Sexual intercourse is a normal physiological process in humans. It is similar to other things we do like eating and sleeping. From attaining puberty
,sexual intercourse can be undertaken til you feel fit. It will not have any side effects,as a fact,it is proved to have so many good effects and protects from cancers.
Weight can be due to varioous factors and sex alone will not cause weight gain.
Virgin females during first intercourse wil have pain during first time but it is only for short time.
I advice you to visit sex health therapist so that you can have a counselling
session so that all your fears are explained and treated properly.
All the best