Probably less than 1%. Sex is both blown out of proportion, and taken too lightly in American culture.
I was born and raised in Long Island, New York. I am 22 years old, will be turning 23 in December. However, my bf of two years recently proposed so I may be losing it at the age of 23, who knows? I was never waiting for marriage, Just real love. I've always been really selective too. I never dated in high school. I've only dated 4 men my entire life and a couple didn't make it pass the first/second date. I do come from a religious family background but I consider myself spiritual and open minded individual. Also, education and family instilling values in me did help me make the choice to wait for so long. I value education a lot and I didn't want to screw anything up. I was actually planning to wait after I received my Master's degree(three more years) but now that I am in love and in a serious committed long term relationship, I may not.
This is just anecdotal and won't help you out with finding percentages or anything, but I'm an American woman who's 23, and I'm still a virgin. I have a couple of female (and some male) friends of the same age, or a bit younger (22) who are still virgins too. I'm not religious or doing the whole "waiting for marriage" thing, but I have been waiting for the "right" guy, which has turned out to be a long wait.
About 40 %
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I think fewer men are virgins at 23 than women. That is because men are generally the "aggressors" who know what they are doing. For women, they wait for the guy to make the move, which most guys won't. But otherwise, the gap isn't that big.