I don't believe in 'I've seen it all', especially as it relates to human sexual practices. I don't see how anyone can. The expression is used when you've seen something so far beyond your realm of experience, you couldn't imagine it if you weren't looking at it. Which is hyperbole anyway, but even if it's exactly true (tricky concept in itself), you've just seen the farthest side of human sexuality and imagination from you. There are countless points between these two disparate sides. No one, but no one, can see it all in the realm of sexuality.
Also, I highly recommend (if you're into discussion about sex and want to start seeing some of those different sides of it: http://sexisnottheenemy.tumblr.com/ )