Lubrication :- As soon as sex impulse takes hold of brain before the action act, there is secretion of fluid( which the young regard as semen) from the penis.
This phenomenon is natural, too for instance, while watching an erotic movie or reading such a book, while talking amorously on phone, when sex thoughts strike the mind and the excessive foreplay& fondling before the intercourse, both the man& woman feel moist due to the secretion within. This is the natural fluid oozing out of local glands.