Is the single most bullshit answer ever. Are you people fucking serious? When has having children ever been a hindrance to entering a field such as medicine, law, or science? Women do all of these jobs without issue. That is the dumbest answer I've ever seen. It's also sexist as hell, I'm a guy and I'm not stupid enough to follow that line of reasoning.
Lack of interest maybe, social stigma, quite possibly, there's also a huge battery of people saying no no no no no don't do STEM, only guys can do it. It's the man's field.
Personally I think it's a lack of parents encouragement. Guys giving up computer time to their sisters, social sexism against women, and a lack of desire due to the above from women themselves. I work for an IT company, it's honestly a hostile workplace. If we ever hired a woman we would get sued to shit for the stuff I hear every day at work.