If the future world of "Lisa's Wedding" is true, then Lisa loses her virginity to Milhouse, strange though that may seem. While she and her mother joke that "Milhouse doens't count," I would have to disagree. The implication in the the episode is that it DID happen.
So then, perhaps everyone could now write the scene, or perhaps a brief story, of the way it happens, either serious or humorous (I expect it will be far more the latter), as short or as long as you prefer (er, no pun intended).
Brad