Our Pit does this all the time. They smell the extra hormones and they will find the dirtiest panties, no matter how big the dirty laundry pile is. Our Boxer doesn't pay them any mind. He's 8...not sure if he knows better or just isn't interested. Our Pit is 4 and he does know better, but he cannot resist. It's also a lesson for my girls that they need to ensure their clothes are picked up and not strewn about like slobs. HAHA
As far as being physically possible...the physiological side of it is the same as humans. Ejaculate is produced by the prostate. Even though testicles are removed in dogs, the prostate isn't. So theoretically yes it's possible. Most of the sex drive is removed since it's the testicles that produce the hormones and drive to mate. But even neutered male dogs do get "erections," but at that point, it's typically excitement not necessarily related to sex.
We dog sat a male Chihuahua mix once who was intact and I felt so bad for this poor dog. It was uncontrollable for him, but I found him cowering in a corner with an erection just ejaculating with no sexual stimulation. He looked as though he was in pain. At the time, all my own dogs were males (neutered) and we were dog sitting him and he was the only intact one. So there was no stimulation at all...it was just something he couldn't help. Poor guy.