There is certainly sexism, but I doubt it is "extremely common", or at least not more common than in other parts of life. Just watch ads on TV - evidently the advertising industry and all their clients are very sexist. That makes every industry.
Why many women complain? I'm no expert, but I would guess (apart from the lack of actual numbers: what percentage of women in IT complain, and what percentage in the average population) there are a couple of reasons. I think some of the issues are issues women face everywhere. Like indecent proposals: is the likelihood higher to get those on a tech event than on other events? But it is easier to point a finger (those IT guys), instead of just "men".
Maybe I can write more tonight, must work now...