I've sold hundred of these and you're the first to have a problem fitting it. It's the same with any threaded element on a car... they corrode and you need to treat them with care before thinking about screwing anything into/out of the thread. If you try and remove a bolt from a rusted thread it's likely, if you don't treat the area with care, that the bolt will shear if you put too much force though it. The same applies in reverse for this.
I had my bumper off, cleaned the thread thoroughly and as a result I can screw my towing eye in by hand.
You need to have cleaned the thread thoroughly if you are at all worried about it being a tight fit.