Being promiscuous is far more of an age thing then it is a gay vs straight thing. I'm in college now, and there is no place more promiscuous then a college campus on the weekend. I have straight friends who sleep with a different girl every weekend, and I have straight friends who have a girlfriend and have never had sex with anyone else. I know gay guys who are sleep around a ton and others who are totally monogamous. So what do they all have in common? They are all young and horny, and the fact that they are gay or straight doesn't make a difference. One thing I realized is that straight people like to put up the facade of being monogamous, but the truth is they cheat like hell behind their partner's back. I have friends who have girlfriends, and when their girlfriends aren't around they are hooking up with other girls. People, especially young people, weren't made to be permanently monogamous. I just think straight people are less willing to openly acknowledge that fact as compared to gay guys who aren't expected to live a certain way necessarily. You can't make sweeping generalizations because the facts just don't support it.
I've been in both situations. I've had periods of one night stands with guys who's names I'll never know, and I've had long periods of monogamy. Does it really matter which life you choose as long as you enjoy it? In my opinion, no. The problem is, we place such a high spiritual and emotional value on sex, when it reality the only point of sex is to satisfy a basic need to reproduce. But somehow people have managed to spin it into some hugely emotional thing, and our culture makes promiscuity a dirty word, even though everyone does it, gay or straight.
So in essence, straight guys are just as promiscuous as gay guys, but they are less willing to admit it because of societal standards.