Here is an image from Wikipedia that shows what Tall is talking about
.
/en.wikipedia.org/wiki/Corpus_cavernosum_penis#/media/File:Illu_penis.jpgif you look at the space occupied by the prostate gland and the
location of the bladder it is hard to imagine that the bladder won't just slump down into the space previously occupied by the prostate. Also, look at how the base of the corpora cavernosa slot in under the pelvic symphysis. Your erectile chambers are pretty firmly anchored in the back. If you get them properly erect the urethra is just going to have to stretch if its too short.