For what it's worth, my tutorial:
I could, a few times with difficulty, during teenage thru early thirties. It was a strain and took preparation, as follows:
Be slender to start with. I was 5'7" and weighed 125 pounds; only 5.5" penis, about halfway penetration.
Have an empty stomach, take a hot shower, do limbering exercises. Sit on a chair, put a shawl around your neck and and pull your head down with shawl ends tucked thru the chair rungs. I managed, even to swallow, but it wasn't really pleasant.
I wouldn't even think of trying now (seventies): damage to vertebrae IS possible.