This is a well known phenomenon and perhaps more prevalent in men than women. Just looking at the 'Falling Shaman' (Lascaux cave painting)http://faculty.gvsu.edu/websterm/Shamans.htm
we can see that this is something which goes hand in hand with the trance experience. However . . . .
This is an excerpt from a teaching given a year or so ago elsewhere and is by no means the whole of the teaching but I post this small part here since it says it way better than I can.
Sex and drugs and rock – or roll down the hill….
Well, you all know the famous Lascaux Bird headed shaman laying in front of the Bison with his staff at hand and a nice big erection.
There are very many more examples in “Rupestrian art” that depict men in partial metamorphosis with erection and also men dancing and…. you get the picture. What in all our oh so plentiful shamanistic literature I to date have found nowhere is the well known fact, that ecstatic trance very often comes in tandem with sexual arousal – especially for men.
Young guys have more to “deal with it” then older guys and women even less and when, it is “softer” and not as disruptive of the trance experience.
In all my years teaching I ever only had 3 serious male students, with whom I worked over longer periods of time face to face. One was/ is a middle-aged MD who took the whole issue in strides and quickly managed to work around it. The next was a young guy from Peru, a Native, who came to Canada for a better life for himself and his new wife and who got “haunted” by the Spirits of his homeland and culture, wanting him to return to where his mom and nana worked as curanderas. He was lost, spoke little English and since I speak Spanish he ended up on my carpet.
I helped him along to come to an agreement and a working relationship with his “home Spirits” and also with the Spirits from here in Ontario, that in short order showed up in his dreams, his first hesitant trance journeys and especially in waking visions, that interfered with him being able to hold down a job and earn an income…..
Learning all that, then all of a sudden he came and said, his wife wanted him to stop being a “brucho” because every time he journeyed he became aroused and – - – oh yes, he had visible erection and the arousal messed up his visions and afterwards he wanted to ….. with her….
Unfortunately most often the theta trance gets aborted by the arousal. There is the powerful Spirit guide – person coming . and he has a blood read round bag that he gives the journeyer and the bag changes into a huuuuge booooob and then the guide changes into a booooby woman and the arousal heightens and since the journeyer is in trance, he touches himself and….. you know where that then goes…..The trance experience deteriorated into “a hell of a wet dream….”
That is one reason for when it happens that in a traditional culture a shaman or apprentice may have a “Spirit Wife” with whom – -….!!!
And he also may or may not have a real partner…..
Some cultures even go so far to stipulate that shamans go through looooong phases of celibacy to then make the wave of ecstatic arousal even more certain and higher and so” ride the wave”…..
It is that, what we see in the pictographs and petroglyphs.
For many guys arousal “opens the gate to trance
And then we could talk about Tantra…..
I hope this helps to explain what you are experiencing.