[flashback to Lily and Marshall in college in 1998:]
Lily: Okay, just forget it.
Marshall: Call me old-fashioned, I just think that I'm the only guy you should see naked.
Lily: Well, then you'll have to do it.
Marshall: Are you kidding me? What if somebody sees it?
Lily: We're not in high school anymore. People don't make fun of you for posing nude for a painting. We're adults now.
[present:]
Barney: We totally saw your butt.