When Harry Met Sally (1989)
Stars: Meg Ryan, Billy Crystal
This is the story of two people who think they're friends but end up realizing they ought to be lovers. The plot keeps referring back to a statement Harry (Crystal) makes early in the film: "Men and women can't be friends because the sex part always gets in the way." This is a classic trick of logic that is destined to lead to sex. In scenario A, the man and the woman aren't friends, so they are free to have sex. In scenario B, the man and the women think they're friends, but they can't be because the sex thing will get in the way, which it does. Conclusion: What the hell, just have sex already.