Yes, I have.
I have masturbated myself in front of my father several times.
I'm used to see my father naked in the house ever since I was little.
Often when he gets back from work, he takes off his clothes and walks
nude around the house. He sometimes takes a nap butt naked.
On my behalf, I'm rarely nude. When that happens, no one is in the house. Of course my dad sees me naked when I'm showering and he enters the bathroom.
But I've never caught him with a hard on or jerking off... :’(
But on the other hand, he has caught me with a hard on a few times, especially when I get off the shower. Since it's normal for men (and young men to have erections), he doesn't mind with it.
One day, I thought I was alone in the house and I decided to jerk off butt naked in my bedroom. When I about hit the "point of no return", suddenly he walks in and catches me. On that moment I looked at him, ejaculating. He stared at me for a few moments, leaving the room without saying a word.
Since my father has a natural attitude towards... well, many things, he didn't get mad.
After that, he has caught me with a hard or even jerking off a few more times (always in my bedroom). At those jerking moments, he stares a little, than usually leaves the room for a few minutes and waits until I'm done, than he comes in again and talks to me or does whatever he has do in my bedroom.