Yesterday's post ended up being a perfect lead-in for this. Spontaneous boners are expected. Guys deal with them all the time. Spontaneous cum? Not so much.
But it does happen to some guys.
Spontaneous Ejaculation (SE, once called SjE).
I covered this way back in 2015 (you can read the post Here). But recent studies are showing that doctors are still somewhat confounded.
It may be expected with an exercise boner as movement of material causes stimulation.
But with no stimulation, it can sneak up and surprise you.
This is not the nocturnal emission, or wet dream, you may have experienced.

This is a for real, non-sexually stimulated ejaculation.
According to the recent article Spontaneous Ejaculation: A Focused Review for Clinicians at PubMed - National Library of Medicine / National Center for Biotechnology, it can be a distressing experience for some guys. Just like premature ejaculation, it's considered "ejaculatory dysfunction." Sadly, the understanding of this particular phenomenon is limited.

"The process of ejaculation has important meanings not only for its association with orgasm but also for the timing to ejaculate in the context of sexual activity. Spontaneous (involuntary) ejaculation (SE) without any sexual stimulation is a distressing symptom. Our understanding of SE is limited. Unfortunately, many physicians are not aware of these cases.
"SE is one of the least reported ejaculatory dysfunctions. The key feature shared in common by these men is SE without any sexual thoughts or fantasies, may be triggered by non-sexual contexts, rarely associated with orgasm or erection. Treatment by psychoanalytic treatment and pharmacotherapy may be helpful. Further research might explore the definite underlying mechanisms."
According to American Journal of Psychiatry, an unexpected ejaculation can occur with severe anxiety.

"The central noradrenergic neurophysiology that anxiety may share with sexual excitement could provide a basis for spontaneous ejaculation during anxiety. The pharmacology of spontaneous ejaculation during opiate withdrawal is used to elaborate this central noradrenergic model."

Women experience unexpected "lady boners," just like guys sprout wood at odd times. Spontaneous orgasms are more common in women, though still rare. However, just like the boner, the difference is that, with men, it's very obvious.