The more you learn about medicine and science, the more you should think, nothing is impossible. Menopause can happen to children, people who've never smoked can get lung cancer, and one in 10,000 people (including actress Catherine O'Hara) have their liver on the left side of their body (situs inversus). It should not surprise you then, to learn that men too can have endometriosis.
Without proper uteri, men clearly don't experience endometriosis in the same way as women. So where then does the endometrial tissue come from? According to a few case reports from the 1980s, endometriosis, or the abnormal location and presentation of endometrial tissue, can arise from remnant uterine tissue in men.
In other words, as the embryo divides in the womb and forms landmark ridges and clefts that separate female from male genetalia (a process that normally occurs between eight and 16 weeks gestation), the possibility exists for remnants of the opposite sex to remain behind.
This is, perhaps, what happened in one of the aforementioned case reports, an elderly man undergoing treatment with TACE (chlorotrianisene), an estrogen drug used to treat prostate cancer. The patient had been taking TACE for over 10 years for an adenocarcinoma of the prostate, when doctors found that he also had an endometrioma (mass of endometrial uterine tissue) of the lower abdominal wall. In the report, doctors denied the possibility of any remnant uterine tissue, but a postmortem exam/autopsy was never performed to confirm their theory.
If left-over uterine tissue was the answer, then perhaps feeding it estrogen through TACE treatment made it a more obvious find. The prostatic utricle, a barely noticeable out pouching in the portion of the urethra that passes through the prostate, is also the obliterated mullarian duct, an embryonic structure that also forms the uterus and fallopian tubes in little girls. So it makes sense that if this never went away (because remember, we all start with the parts of little girls), a common complication of female genital anatomy, endometriosis, is entirely possible.