Leonard Cohen's "Hallelujah" is the backing track to a love scene in Watchmen. Although considering it a sexual song is not a novel concept, there is now a very specific scene which I will forever associate with it.
• "Rock Lobster" - B-52's (Knocked Up)
— Near as I can tell, this song is gibberish (unless a "rock lobster" is some weird phallic reference), but it plays when Seth Rogen performs the titular act upon Katherine Heigl.
• "Faithfully" - Journey (Talladega Nights: The Ballad of Ricky Bobby)
— Back before anyone was enchanted[1] by Amy Adams, she delivered one of my favourite movie-motivation speeches to Will Ferrell as Journey's "Faithfully" demi-diagetically plays in the background. It's not so much that this isn't a love song, but what Ferell and Adams do during it is...not quite romantic.
• "Street Lights" - kanYe ('Grey's Anatomy')
— kanYe's lamentations on his inability to reach a point in life that he's already paid his dues at which to arrive becomes an excuse for Sandra Oh to impersonate Marilyn Monroe.
• "Crazy On You" - Heart (Harold & Kumar Go to White Castle)
— Unlike "Faithfully," the manic meter of "Crazy on You" matches the fervor of the...affection...of this pair The scene merely happens to feature a gigantic, anthropmorphic bag of weed. One can debate whether more obvious choices would have been better.
• Honourable mention: "How To Say Goodbye" - Paul Tiernan (Nick & Norah's Infinite Playlist)
[1] No level of irony can mitigate how horrible that was, so I will just apologize. (._.)
Wherein unreasonably free time is dedicated to proving Jonah Hill is funnier than you.