"One time we were at a dinner party where everyone was over 60 and, before we knew it, we realized that everyone at the table had gone outside to smoke pot and they didn't invite us. That basically says it all," Antonoff, who is the guitarist for the band Fun., tells New York.
The musician, 30, says he and the Girls star, 28, aren't big partiers, and a recent spa weekend helped them to establish that they prefer to chill out.
"Everyone else was 56, and like, either coming off a divorce or trying to dry out. We were like, 'This is great! We love being here!' "
The mellow couple met on a blind date set up by Antonoff's sister and comedian Mike Birbiglia, and the first date went so well, the guitarist quickly told Dunham everything about himself, "because when you really like someone, you want them to know everything about you," he says.
Antonoff, who also fronts the band Bleachers, says he's not quite ready for marriage, but the idea of having kids intrigues him.
"It's hard to imagine when the right time is, because things are so crazy at the moment," he says. "It just seems like the most fun thing in the world. I've never met people who have kids who haven't looked me in the eye and been like, 'It's the greatest thing that's ever happened.' "