Hormone

sex will trigger hormone growth.
sex with BBC will accelerate things more.
not having sex will lower hormones rate can be negative.
hormones can affect things like boobs, nipples, hips, waist, thighs, and more on female; dick size, muscle for males.
will be done and managed in story cycle that will run on weekly basis. lowering/increasing hormones variable depending on the acceleration variable gained from having sex. ving sex.