I think this is actually a controversial thing to say, but I've always pretty much assumed that women who wanted sex could have it essentially whenever they wanted (or to be less absolute, 90% of women), whereas men wanted it but found it unattainable (or, again, say, 90% of men). So I always figure if women aren't, it's because they don't want to and if men aren't, it's because women don't want to.