What Williamson said was that if you personally want to do something about abortion, make your own personal statement by not wearing un-feminine clothes. If you personally were to wear only skirts and dresses, you would be making a statement about femininity and your nature as a woman -- that it is intrinsically different from a man's. This is a positive step towards a culture where the murder of the unborn is unthinkable.
Cardinal Siri of Genoa (who twice was the leading candidate for pope) wrote the following excellent essay on the problem of women wearing men's clothes. He wrote this before abortion became widespread, which proves how prescient he was:
NOTIFICATION CONCERNING MEN'S DRESS WORN BY WOMEN