This is a true story. Onan's Razor is a conflation of Occam's Razor and Onan from the Bible who spilled his seed. Onanism is a synonym for masturbation. I also use a razor to keep myself trim, in lieu of a diet.
When I was just about 16, my dad and I made a really nice room for me in the basement of our old house, with carpeting, paneling, dropped ceiling, old furniture, bookshelves, a nice stereo, etc. Very 70's, and much like my own little apartment. (Before that I shared a small room with my younger brother, so this was my first taste of true privacy, or so I thought.)
Right after I moved into that room, I'd just had a long hot bath, and I went down there wrapped in a towel to change and get ready for bed.
Now this was back in the dark ages when to enjoy porn you had to flip pages that stuck together just from the static and there was apparently no such thing as hardcore without going to France or if you were 18, an X-rated movie. So I got out a Playboy from my small hidden stash, and spread out on the bed to air-dry and peruse the material.
Long story short, I'm getting well along with this little project and I'm stroking away with a fully rock-hard nearly six inch erection when my Mom and brother come strolling through the door. (It didn't have a lock, I think with all the expense of the remodel Dad was saving the $2 difference between the plain lockset and the one with the little locking knob, why would it matter when I was down in the basement?)
So there I am, dick in hand, inflagrante delicto, stretched out buck-naked on top of my made bed with a Playboy open to a girl with her butt in the air. Hmmm.
Mom says, 'Well, I guess you're playing with yourself.' and I say, lamely, 'No, I was just drying off and looking at this Playboy.' (At this point I'm on the edge of the bed curled over my knees like Rodin's 'The Thinker' trying vainly to hide my raging hard-on, but it is pretty dang obvious what's going on.)
Then she says, 'Well, don't worry about this, we'll all knock before coming in here from now on.'
Believe me, I installed a lock, but that was still pretty cool how my Mom handled it, and in that strange 'getting caught is a turn-on' way, it still kinda turns me on to remember that night.
As I recall, I did go ahead and finish almost immediately after they left, I can't imagine that I didn't, but if not then, I did for sure when I went to bed for real. I can even imagine that I finished when they left and then did it again on the strength of that getting caught vibe.