My favorite poly song is "My Boyfriend's Girlfriend Isn't Me" by Must Be Tuesday - always turn it up loud when it comes around on my MP3 mix that MrS made for me = not really a love song though. (In my mind it's the polyamory "Theme Song"
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(ETA - NOT the same song as "My Girlfriend's Boyfriend Isn't Me" by Autograph, which may be a fine song for those who enjoy music that I can't understand the words to...)
I also like Jane's Addiction "Three Days" - more of a sex song than a love song though...