Yuri is the purest form of love
The fact that two women who love each other (yuri) is so pure and wholesome that it's the purest thing in the world. It's used when seeing yuri couples in anime or manga.
Guy 1: (watches two girls holding hands) How can they be so cute together?
Guy 2: Didn't you know that yuri is the purest form of love?
Guy 1: No I didn't.
Guy 2: Well, now you know.
Guy 2: Didn't you know that yuri is the purest form of love?
Guy 1: No I didn't.
Guy 2: Well, now you know.