During sex the female's vagina produces prostaglandins (E1 & E2) counteracting the males loss of after ejaculation.
Masturbation will result in prostaglandins being lost and will not be refreshed by the female's vagina.
High levels of prostaglandin D2 causes hair loss.
Other types of prostaglandins cause hair growth.
In hair growth (eyelashes ad eyebrows), prostaglandin affects two phases of the growth cycle. It increases the length as well as the number of hair follicles.
Bimatoprost increases the hair growth prostaglandin (in eyelashes & eyebrows).