That's happened to me before too. It's surprisingly easy for a post to get flagged as "adult." I think it might be automated, so the system probably isn't perfect. I think I remember there being guidelines for thread creation that indicate what type of content gets things moved to the "Adult" section...but I've also been a part of many forums over the years, so memories might be blending together. Regardless, I can't find the said guidelines. If anyone wants to chip in at this point and link the page, that might be useful.
On another note, I doubt that it has anything to do with you. I'm certain that it wasn't personal, especially since I'm pretty sure that moving posts to the adult section is automated. This is a fairly open and accepting community in my experience, so--at the risk of sounding like a broken record--I doubt it's you.
Now it's time for me to go look at that the said post. All the hub-bub has piqued my interest!