I'm leading a student tour of Madrid, Paris and Rome this coming spring break. I've been to Europe fifteen or twenty times, and, though this time, I will have kids to look after, I will toss in my two cents.
London and Edinburgh rock for just about everything, and Dublin's not far behind.
The women are beautiful in Paris, but even more so in Slovakia.
Amsterdam blows. If you want to get fucked up, or fucked for pay, fine, but otherwise, there are far better places in the Netherlands.
Somebody mentioned Prague. Good choice, but don't forget her bigger sister, Budapest, where people are friendly and history is vibrant. My family is from Hungary, so I am biased, but it really is a gorgeous place. If you go, you have to go to the big baths, and I can suggest a few very cool bars to check out.
Italy and Spain are beautiful, though at that time of year, they won't be exactly "beachy." You may want to consider the Greek Islands, too. Santorini has a particular reputation for sex. At any rate, stay away from Athens for a while yet.
The suggestion of Iceland was a good one. Icelandic women are not only hot, but sexually liberated and oftimes even sexually assertive.