Yes, by all means, it is upsetting. I've had it happen, except it was a song. The song was recorded and published under someone else's name, who also registered the copyright and ultimately collected the royalties. I learned a lesson.
Register your copyrights, "publish" your work with as many non-relative witnesses as possible, and publicize your ownership.
Your voice on this poem has not actually been stolen since you have taken action. Follow it through. It's a healing experience for survivors to defend their boundaries.
Register that poem and all the others today. It will be your own New Year's gift.
I have taken to the stand-up comedy stage to educate other male survivors and those who try to love them. I blog about my isolated religious upbringing where physical and sexual abuse were commonplace and I serve as a facilitator of a weekly support group for men who have suffered sexual assault.