I think it's flawed research- it was a tiny sample of women ( just over 2K) done in the 1990s, and it doesn't address the FACT that early menopause is hereditary. So are all these grandmothers, mothers and daughters not having sex?
Surely the easiest way to prove a theory like this would be to do a study on nuns.
I also don't agree that there reason for menopause is unknown; most meno experts will say it's when the woman's body runs out of eggs.