There is a well known gemara in Sanhedrin Daf 101a that says
??? ???? ????? ???? ?? ??? ?????? ????? ???? ???? ??? ?????? ???? ???? ?????? ??? ???? ???? ??? ????? ???? ?????? ????? ?? ?????? ???? ???”? ?????? ????? ????? ?? ???? ?????? ???? ????? ??????? ?? ???? – Someone who sings pesukim of shir hashirim the way leitzim do make the torah dress in sackcloth and cry out to hashem and say your children etc. Rashi explains that it goes on the whole torah not just shir hashirim that it can not be used to make songs at a party. The Achronim are all maarich on this in siman 510.
But the Likutei Maharil and the sefer Chasidim 147 say another pshat. They say it is only going on shir hashirim and when someone sings the words of shir hashirim for a song than the torah etc. The words of shir hashirim may appear romantic, but it is assur to make them into a shir agovim . They have a very deep meaning and it is assur to use it literally. He says that is why it says leitzim. Because leitzim sing these tyoe of songs………