Before we go any farther in this discussion, I would like to state that I am female. Not that I am speaking for all females (no one can), but I think that it is important that you know where I am coming from. I also have a young daughter and would like the world to be a better place for her, as all parents do. It is also worth noting that the title for this post was inspired by one of my favorite movies: This is Spinal Tap. The band wants to release an album called Smell the Glove with a cover of a naked woman who is leashed and is forced to smell a glove. The lead singer mistakes the word "sexist" for "sexy". Hilarity ensues.
The name and picture of that fictional album is undeniably sexist and degrading to women. But does that apply to the in-game actions of forcing bunny ears on appropriately "aged" females? Sutro at Feministing.com seems to think so (as do a few of our readers who wrote in about this topic). He calls this achievement "unconscionable", which is a pretty strong word.
Without a doubt, putting the ears only on female characters and having them be the equivalent of legal age to pose nude is a reference to Playboy. And Playboy, as most adults know, is a men's magazine with naked women inside AKA smut or porn. Many people believe that all porn objectifies and is otherwise demeaning to women, even if considered tasteful as many consider Playboy to be.
But while the female characters have no choice as to wearing the bunny ears, which Sutro says makes many female players feel uncomfortable in real life, they are not stripped down into a Playboy Bunny outfit or otherwise affected in any other way. So this really comes down to knowing what the reference is and being bothered by it.
On the one hand, anyone who doesn't know it's a Playboy reference, such as children, just think it's fun to put bunny ears on other players like they forced pumpkins on players' heads during Hallow's End. It is just bunny ears to them and is innocent fun. Other people enjoy the pop culture reference and also mean no harm when participating in the event.
Personally, I wish they had left the gender and level restrictions out of the achievement. Being "forced" to wear bunny ears doesn't bother me at all and I am having a good time in Noblegarden, but I don't think this was the smartest decision Blizzard has ever made. (Edited to add: I tried to present both sides of the topic here, but if you are interested in my full, unbridled opinion, I've expounded on my personal blog.)