Using the General Social Survey, I discovered that 5.8% of male respondents born in the U.S. are virgins. But 7.2% of men born under the sign of Sagittarius are vigins, and 7.0% of Scorpios are virgins.
Have I discovered that there really is something to this astrology thing? I don't think so.
Sagittarii are born between November 23rd and December 22nd. Many school systems have a grade cutoff of December 31st. Children born January 1st or after will be in a lower grade in those school systems. This means that Sagitarrii are likely to be the youngest children in their grade, and thus the least mature physically, emotionally, and intellectually. This puts them at a disadvantage relative to their older classmates and more likely to grow up lacking the social skills necessary to lose their virginity. This is the same effect as the findings about adolescent height and labor market outcomes.
Scorpios are born between October 24th and Novermber 22nd, which explains why they are also more likely to be virgins.
On the other side of the cutoff date, only 5.4% of Aquarii (born January 20th to February 19th) and 4.7% of Pisces (born February 20th to March 20th) are virgins.
The conclusion is that there may be some social benefit to having your male children be among the older children in their grade at school. (This creates a negative sum game when parents try to game the system and get their kids held back a grade. 20% of children will always be the youngest 20% in their grade, and it's not in society's interests for people's entrance into the workforce to be delayed by a year.)
There has been some confusion in the comments about the cutoff date. The most common cutoff date today is actually September 1, but in the past the most common cutoff dates were 12/1, 12/31 and 1/1, and the later cutoff dates probably apply to most of the GSS respondents.
Here is a research paper on this very issue, and there is a chart at the end of the paper showing the years that states changed their cutoff dates.