allegedly--in colonial america, there were "courting candles" and a male caller was invited to stay as long as the candle was lit-- such courting candle might be the source of the term love lights.

but -- i have only seen courting candles in catalogs that sell candles and candle holders-- so i don't know if they existed-- or if its all just a marketing ploy.

There is the song- Let Me Call You SweetHeart with the lines
"Keep your love light burning, in your heart, so true,
Let me call you sweetheart, I'm in love with you"

which suggest a love light is not a real light, but more like "burning desire"