What makes an anime a "waifu anime"

I've seen people label animes like to Love Ru as "waifu anime", but I don't think I understand the pattern. Is it just an anime based around a girl and a guy, is it the "prolonged virginity trope" that all these anime have, does it have to be a harem or some subset of a harem, or is it just a general term for any anime some subset of people find attractive.