That's not common, but far from unheard of, and it's definitely possible to have sex like that. Google "coverage index foreskin" to see pictures of how different foreskins can be. Some (circumcised) men restoring their foreskins aim to have full coverage when erect.
You should be able to retract though. If you can't, then you should probably see a doctor. I think it's usual for most men's foreskins to retract automatically when they're erect.
If it works, it works. Not every man's foreskin is long enough to cover the glans entirely during an erection, so your experience may not be as common - but for what it's worth, I'd say it sounds normal.