The crooked of mind do not prosper,
and the perverse of tongue fall into calamity. Proverbs 17:20

There's no commandment not to masturbate, sure. But it isn't wise unless it isn't "crooked." What wouldn't be "crooked?" Past sexual encounters that involved normal sex. That's it. Or possibly you could find a girl that is alright with you thinking about her but you'd have to ask her for permission.

But looking at other people having sex is perverse. It just is. If you were to not look at porn for awhile and then came back to what you watched, you would be horrified. You've become desensitized to it, more than likely.

I suppose if you found girls that posed nude and explicitly said that they didn't mind if guys thought about them when looking at the pictures, that'd be alright too. Maybe do some research?