"Pacific Overtures"
It's a tough question, even in shows where you really don't think that there can be a romance plot, one gets worked in somehow. I was going to answer "1176", but you have John Adams' and Thomas Jefferson's wives singing and both sing romantic songs.
Even the flop musical "Billy", based on Herman Melville's "Billy Budd", worked in a flashback so that Billy and his girl could sing a love song.
"A coherent existance after so many years of muddle" - Desiree' Armfelt, A Little Night Music
"Life keeps happening everyday, Say Yes" - 70, Girls, 70
"Life is what you do while you're waiting to die" - Zorba