I would say it's a pride game for a lot men. Funny thing is, once you really do get "out there" and are one amongst the competition or the "herd", you realize very well that when dudes start using sex or talk of girls to sound bigger, they're actually weaker. ANYONE who has to use sex to power themselves is weak. Why? Because sex is something that we can all get and do. It's not hard. SO, not only are you taking credibility for something that can be attained by so many, but you are also saying that something else within you is lacking forcing you to bring this up. Insecurities, as killah stated.
Should you speak of sex with others....depends. You pretty much have to think if it's absolutely necessary that someone else has to know you hit that shit. Otherwise, to me, it's like taking a shit or eating food. It's something that humans do, we know it goes on behind closed doors...we can assume you do or dont. But we dont HAVE to know. Infact, your better off to keep your mouth shut. Modesty will get you even further AND when you keep some of your cards hidden, you keep people guessing about you. Mystery is appeal aswell.