Strange but I had thought just the same as Lyn regarding poos and additionally there are some gay men who use the rectum for pleasure,(no intended criticism). Really there should not be a problem in inserting the needles. I didn't have any problem in this respect and without anesthetic for my TRUS biopsy back in 2007. There was a little sting each time the core was taken rather like being flicked with a rubber band and a noise comparable with the 'click' from an office stapler.
I do not wish to add to your concerns but need to be honest. Even the best MRI scans can sometimes fail to show present cancer. TRUSS biopsies sometimes miss cancer and where oncologists nevertheless feel strongly it is present, may recommend a second biopsy. On very rare occasions this has not shown anything and it has taken a third biopsy, likely a Template one, to find any significant cancer but the chances are very much that you will not be subjected to this. However, it does show with the best will in the world how the tools available to consultants sometimes make it difficult to arrive at an accurate diagnosis.
It is of course the prerogative of a man to refuse tests or treatment at any point he so decides. Any PCa he has may never become a problem and he may die of something else. On the other hand his cancer may advance so he faces a rotten death and wish he had gone through with tests and treatment when there was a good chance of cure or long remission.