Jaclyn Friedman has written a brilliant piece in the Washington Post, explaining (again) the beauty of affirmative consent as a model and a practice for behavior, and as policy when considering sexual assault cases.
One of the many points Jaclyn raises is that while some older people are all up in arms about the idea of affirmative consent, many young people get it, like it, and in fact are quite relieved to see it becoming the norm.
Why? A couple quotations from Jaclyn's article helps to spell it out. But really, the best thing for you to do is just read the whole article.
And that's the bottom line to a large degree. Our culture, and the acculturation and socialization many of us who are over 40 and in positions of policy and law making lived through, told us that talking about sex is wrong, bad, and just not done. Sex is "supposed" to happen in the dark, with no real communication. The idea of affirmative consent shakes that belief to the core. And it's about time.