I read some interesting statistics on the sexual behaviour of

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I read some interesting statistics on the sexual behaviour of girls in the Yanomamo tribe a few days ago.

In Yanomamo societies a girl is forbidden to have sex before menarche which usually happens at about 13-14 years of age. But the anthropologist Chagnon found that 2/3 of girls become sexually active before then at about 12 years of age.

It looks like these rules against sex before menarche are just as much social inventions as the age of consent in our societies, and just like in our societies they are often broken. It seems that girls have evolved to become sexually active slightly before menarche and the beginning of their fertility, which occurs about 2-3 years afterwards.

Why is this? What's the evolutionary purpose in being sexually active (and attractive) before becoming fertile? Is it something to do with the fact we form long-term relationships and that girls are often chosen as wives several years before they reach reproductive age? Maybe being sexually available and attractive gets male attention and incites them to compete for the females?