Who dicided that girls could participate in the Ancient Olympics?

Answer 1

It's not really that anybody sat down and decided that women couldn't be olympic athletes, but in ancient Greece, as in much of ancient Europe, women were regarded as the less athletic of the two genders. Ancient civilizations conformed to strict gender roles, in which men competed to be the strongest, fastest, manliest, most eligible male, and women were expected to be the "property" of their husbands, fathers and sons. It was almost unthinkable that a women was even capable of competing in the events, and it never would have been allowed.

Only in (relatively) recent times, as gender roles are being challenged, have female athletes began to rise.