A man's virility is judged in the public eye. A woman's is not. This may sound counter-intuitive given that there are departments stores with the entire first two floors devoted to women's beauty aids yet it's so.
Our media, legit or pornographic, speaks of highly-muscled men with full heads of hair ramming ten inch cocks into women who then choke on the gallons of semen men release within them. It's this TomOfFinlandesque image of male sexuality that pervades porn, straight or gay.
Men always feel judged either by peers, women, employers, and definitely, ourselves. We are constantly in a state of wondering if we're doing the right thing even if we say, and act, like it's nothing at all. To appear effortless in action is an admirable skill.
Being male is about being more. Be bigger, faster, richer, better, smarter, than the other man to compete with them, to be more of a man. We're just programmed this way. It's how every other male of every other species gets to spread his genes around and that same need to be better applies.
Even deeper than this is what semen is to a male. Women give birth, men just drop a load. When a man ejaculates with a partner it's the ultimate expression of his virility. It's what we're here for, why masculinity exists in the first place. More semen means more sperm which means more masculinity. The ubermale ejaculates copiously and with great force because he's a fucking MAN damnit and a man's cock is a gun. It's a gun in that brings women (or other men) to ecstasy, and a gun that fires life instead of death. Semen is our essence, our maleness and like everything else male, more is better.