Slept on the right side of a queen mattress for 22 years of marriage. Now been divorced for about a year. For the first 6 months or so, i stayed pretty much on the right side, Then, for some reason I don’t recall, I moved my phone charger to the other nightstand and began sleeping on the left. One of my dogs has now adopted the right side.
I prefer to sleep on the side nearest the door. I don’t know why. Regardless of whether there is male company or not.
Tallish, single, queen bed and I sleep all over the place.
Sometimes diagonal, over onto one side, back to the other, I’d be a dead set PITA to sleep with.
Full size bed. I start off in the middle, sprawled out on my back with the pillows overlapped just so. Then, sometime during the night, the cat comes in and sprawls out horizontally on the left side so I tend to wake up clinging to 6" of mattress on the right side for dear life.
Near one side. Just easier to get in and out.
I start out slightly to the left of the middle. But with two large, long-legged dogs who insist on spreading their legs out to the full length and pushing them against my back during the night, I frequently wake up just at the left edge of the bed.
They are clever enough not to get up on the bed until after I’m fully asleep. Otherwise they would be evicted from the bed because of their bed-hogging tendencies.
^This, exactly!
I almost always sleep on one side and am not a restless sleeper. I can, and have slept, with stuff on the other side of the bed and not disturb it at all.
I also shamelessly hang my feet or arms over the edge. I am scared of nothing! I was sleeping alone, in the full dark, before I was four years old. My dad abandoned me and my mom was…a pretty neglectful mom, so my family jokes that I knew I had to be tough and independent pretty much from day 1. 
I sleep by myself in a queen and I sleep toward the right as you look toward the headboard. This is mostly because it’s closer to my alarm and I need to be able to turn off that noise from hell without delay. I do sometimes force myself to move to the middle, but never to the left.
I rotate my mattress in spring and fall in an attempt to wear it out evenly.
The “monsters” here are the four-legged furry canine kind who will lick your extended hand/foot, or, worse, touch it with a wet nose. That will make you sit bolt upright.
One of my cats walks across my headboard, and this morning she woke me up with the softest, most delicate touch of her paw on my upstretched hand to tell me it was time to let her out.
I sleep balanced on the edge of the bed with one foot hanging off.
I have no idea why.