2) There is semen on the dress, but it's not Clinton's. Monica, in her usual mendacious way, lied to Tripp about it.
3) FBI agents leaked the existence of the dress as a favor to Secret Service agents. They wanted to signal the agents that there is plenty of damning evidence of an affair, so that the agents would know not to lie to protect Clinton.
Wag the Dog Theories
There are, believe it or not, four separate scenarios suggesting that Clinton may use a war abroad to distract attention from his war at home. Kosovo, Iraq (thanks to artfully timed obstinacy by Saddam Hussein), and India-Pakistan are subjects of the widest speculation. My favorite Wag the Dog target is St. Kitts, a Caribbean island where a local drug dealer (Charles "Little Nut" Miller) has threatened to murder American students at the island's veterinary school if the United States tries to extradite him. St. Kitts = Clinton's Grenada.
The Hillary Theories
1) Clinton wants to apologize but is afraid to because he never told Hillary about Monica. (She did not mind his Gennifer Flowers affair, because she knew about it.) Clinton fears she will hate/divorce/abandon/kill him if he admits the affair.
2) Very similar to "Hillary Theory 1": Hillary won't let him apologize because she has spent so much of her own credibility defending him ("vast right-wing conspiracy," etc.). She refuses to be made to look like a gullible fool.
The Right-Wing Conspiracy Theory
Lewinsky belonged to a devious, long-term Republican conspiracy with, as one advocate of the theory puts it, "instructions to orally seduce the president in order to assure positive transfer of his semen (positive proof of sexual acts) on an article of her clothing." Thus she kept the dress, knowing that her affair was going to be made public by her good friend Linda Tripp months later, and she would have a chance to produce the dress as evidence.
The Non-Right-Wing Conspiracy Theory
Last fall (months before her existence was revealed to the public), a Secret Service officer warned Bob Dole to avoid a neighbor in the Watergate named Monica Lewinsky. The officer told Dole that Lewinsky and Clinton had had an affair and that Dole should take special care not to be seen or photographed with her, because it would lend credence to theories that the affair is a right-wing conspiracy and that Monica was a bimbo plant.