Getbig can be un female friendly at times, so it gives an appearance of less female fans if you go by this board. Women are the minority of the 'fan base', but we are around.
Many of the shows have female competitions, as well. The women who compete bring their friends and families. That accounts for much of the female attendance, plus, the girlfriends and female family members of the male competitors.
A better test of bodybuilding's popularity among women would be the percentage of the readers of bodybuilding mags like M&F, Flex and MuscleMag who are women. I would bet it's under 5%.