Ken Lammers has an interesting series of posts on various laws governing sex, including this interesting post on consent in the U.K. Consent is a huge issue in many sexual assault cases; and in "date rape" cases, it's the only issue. Many states (including Illinois) require the prosecution to prove two elements of consent: 1) that the victim did not consent, and 2) that the defendant actually knew the victim didn't consent. The first element is easy enough to prove, as all the complaining witness need say is, "I did not consent." The second element is more difficult to prove, and is what keeps many people wrongly accused of date rape rape from being convicted.
In some jurisdictions (if memory serves me, New York and it now seems, England) the prosecution need only prove that a reasonable person in the defendant's shoes would have known that the complaining witness did not consent. Basically, it turns a rape case into a negligence action. In negligence states, the prosecution can more easily prove its case, since the prosecutor doesn't have to prove that the defendant intended to rape anyone. He just has to show that the defendant acted unreasonably. Get a jury who thinks that sex with someone after a couple of cocktails is per se unreasonable and you can see the potential problems.
The sad thing about date rape cases is this: America is sexually puritanical. The only way many people can relax enough to have sex is after a couple of drinks. Should people not have sex after they've had a couple of drinks? Whether or not you've thought of it this way, in some states their sexual assault laws criminalize drinking-and-boinking. After all, if someone falsely accused you of something in a negligence state, do you really think you could beat the charge? In an era of mandatory registration, would you even want to fight the charge? Any good lawyer would tell you to cop a plea to anything that would keep you off the registristry. Anyone other than St. Thomas More would accept such a deal, his or her own wrongdoing notwithstanding.
These days, having casual, drunken sex is extremely stupid because not only does one risk getting an STD or an unplanned pregnancy, but one also faces rape potential charges. Some parents have a double standard when setting sexual boundries for their sons and daughters. Not me - I'll keep 'em both up in check. After all, there are lots of people like Kristy Holden in the world.