Peeing with a boner is not only possible but fairly easy for some including me. The problem is if I tried to do it at a urinal (where I would never have a boner anyway) the stream wouldn't hit the urinal it would hit the ceiling.
For the record this entire topic is a bit off topic for a nudist forum.
When I was much younger it seemed like I always got a pee hardon and it was nothing to head to the urinal to relieve myself. At times I had to stand back a bit or pee on the wall and I can never remember a time when others peeing at the same time had an adverse reaction. Oh sweet memories of days long past!
Well, I have to admit that when taking a leek in a public lavatory I do check out other men's penises. I have seen some real whoppers but most are about the same as my own. Not being a homosexual, I am not generally aroused by these sights, generally, but being, in fact I suppose, bisexual, some are very appealing. That said, I am there to relieve myself and not solicit sex so I finish my business, zip up and go on my way.
I would often get a semi whilst at the urinal, rarely a full boner. It's the thought that other men along the line may be looking at my dick that causes my arousal. I do have a furtive look at other guys. I have noticed that some guys linger at the urinal after they have finished peeing, spending a lot of time milking and shaking their dicks. It's a bonus when the guy next to me flops out a whopper!