Why affirmative consent policies are a bad idea

I know, I know. I've written a lot of blog posts about ending rape culture and such. And I have not stopped having those opinions. But the idea that students should sign contracts before engaging in any sexual activity is ridiculous, and here's why:

1. Sex is something you should be able to back out of either before or during the sexual activity without negative consequences. Contracts don't work like that. I thought more anti-rape activists would have that criticism, in light of how contracts were abused in Fifty Shades of Grey. But apparently not.
2. Under such a policy, if someone is accused of rape, they are guilty until proven innocent. Now, I know that rape often goes unreported and that it's difficult to prove, but taking a side before you know anything can lead to many negative consequences. Rape is a horrible thing, even more horrible than being falsely accused of such a crime (assuming the punishment for rape isn't death or torture), but jumping to conclusions is also a bad idea. In a psychological setting (a counselor's office, for example), if someone tells you they were raped, you should believe them and take them seriously. In the legal process, both sides should be examined.
3. It's indiscriminate, so it also invades the privacy of sexual decisions between consenting adults. If it's consensual, it's none of your business!
4. It would involve a lot of trouble only to do more harm than good. 

Rape is horrible, and we should find better ways to prevent it.