How can I talk to a girl after I've seen her peeing?

I'm really attracted to this girl, and I wanted to ask her out. But then last night I was at a party and I walked into the bathroom and she was sitting there on the toilet in mid stream. I saw everything. It was her fault because she didn't lock the door, but I still feel pretty awkward about it. I'm still attracted to her though and I still want to ask her out. What's best? Should I pretend it didn't happen? Or should I try to make a joke out of it?