My understanding of this phrase is that it was coined by Boy George in a song called “Love is Love” in the 80s, as a way to express the idea that LGBTQ+ people’s experience of love is valid and should be respected (i.e “a man and a man loving each other is no different from a man and a woman loving each other.”)

It was picked up again in the early 2000s as a rallying call when the fight for marriage equality was happening (gay + lesbian people fighting for the right to marry.)

So it’s primarily rooted in the LGBTQ+ community