I've almost walked in on my brother masturbating.
I once walked into the living room and saw my brother and his girlfriend making out. Ewww. (For the record, his girlfriend wasn't that pretty and had a little too much butt BMI).
i walked into the bathroom and found my sister masterbating *shudder*
Also, pics or it didn't happen.
Either way, it's completely natural and a private thing. If you don't want to see it then its fine, you don't have to. Just turn around, walk away and forget it. [/mrlevinstein]