The thing is that examples proving your roommate wrong are everywhere, literally from Nicki Minaj to Iggy Azalea to M.I.A. to Courtney Barnett. But if he wants to believe that all female artists sing about is love he is going to zone in on the love hits/songs of each album and ignore the rest. You wouldn't off the top of your head think "Nicki Minaj doesn't sing about love" but she raps/sings about how she's a badass plenty. Is sex love?
St. Vincent - and check out her album with David Byrne - or does Byrne's masculinity negate her contribution? The Kopecky Family Band (two lead singers one male one female) I see a lot of blues artists have shown up in this thread but we've neglected modern ones: what about Regina Spektor (no relation to Phil)?
like even Britney Spears sings songs that aren't about love/relationships, see her song "Lucky" for instance or (if it counts) she covered the Rolling Stone's "(Can't Get No) Satisfaction" and that's not about love; lust at most, and I think the song can be interpreted on a societal, not interpersonal, basis
the kills fuck even the spice girls : "wannabe" is as much about female friendships as it is about potential relationships; they sang a song about mothers for goodness sake what about 4 non blondes whose hit song is political what about the fucking cranberries