Women have participated in all the olympic games except the first one in 1898. Since then the percentage of women taking part has grown every year except 1904 (St Louis) and 1936; when the Nazis (Was für eine Überraschung!) managed to reduce the percentage of sports women who competed at the Berlin olympics.
Apart from that historical blip the ratio of men:women has got more and more balanced every year.
Highlights for gender equalty are 1976 in Montreal, when the percentage jumped a record 5.4% (from 15.3% (Munich) to 20.7%) and again in 1996 in Atlanta, when the percentage grew 5.2% from 28.9% at Barcelona to 34.1%.
Since Atlanta the percentage of women to men has continued to grow, but more slowly. Since 2000 (Sydney) the growth has averaged just 2.1%. London 2012 reached almoist 55:45 men:women.
If this trend continues at the same rate it means that parity will be reached in 2024. Though I suspect that all bets will be off if Doha or Dubai host.
(N.B: the data come from the usually excellent Wikipedia – if anyone knows of a better source please let me know)