If you want to hear from a man's point of view, here is mine. It is only mine, and may vary from other mens' experience, but I know it has been my experience for about 35 years of sexual activity (I'm around 50).
As I get sexually and psychologically stimulated, the glands inside my body start producing an absolutely clear lubricant that works its way down to the "bulb" at the base of the penis that is located inside, just between the scrotum and the anus. By pressing this area and "working" (almost "milking") it toward the tip of the penis, it can be brought into the penis and out the end of the urethra. Tightening and releasing the pubocoxcygeus muscle (like a Kegel exercise) might help it to make progress. It is the best lubricant I've ever found for sex, especially when mixed with a little saliva.
Since this lubricant is NOT propelled out with any muscular spasms, there is not necessarily any "moment" that a man might notice it, especially if distracted by the amazing beauty and sexuality of a woman! However, I WILL say that I am usually cognizant of the buildup of this lubricating fluid, but I am usually looking for it, so I can use it on my penis or fingers depending on whatever we might be doing.
As PenguinsRus pointed out, you DO need to worry about sperm getting involved in this lubricating fluid, as the male's glands and storage areas inside are very closely involved and connected with each other, so you CANNOT BE SURE anything that makes its way out of the penis is sperm-free!
