No... I don't think that the amulet is meant as protection against impropriety. The amulet is, after all, voluntary. The whole point of Araminta's exile to the colonies is that her family is sick of her improper ways. The maids, the chaperone, and (to a lesser extent) the eunuch were all partly protection against a consensual encounter (have you ever tried to get it on with a eunuch looking at you? Totally kills the mood...) (ok, the eunuch at least was definitely also protection against a nonconsensual encounter... but I think he did double duty as part of her generally awkward disapproving entourage). Her family wasn't just protecting her, the were trying to control her... but they were also trying to protect her from what they saw as a clear potential for danger.
When Araminta was given the amulet, they didn't say "if you ever feel like you are about to consent to sex, put this on so you'll become a dude! That will make it ok." They said "put this on if the worst is about to happen." And for that matter, they gave it to her to put on when she felt like it. They didn't weld it to her neck and put a spell on it so if she ever got sufficiently turned on she would turn into a dude! The amulet was definitely a way to prevent her from being hurt.
Though, come to think of it, that would make a pretty hilarious premise for a fantasy rom-com. A well-intentioned but hopelessly backwards magician "blesses" a princess to become a prince whenever she's turned on, and she spends the entire story trying to hide this from the guy she's into (possibly while having confusing encounters as a guy on the side). Only finally when he discovers it, it turns out that this prince charming is bisexual and totally into boy-her also! And true love's first kiss breaks the spell. Because having your parts turn inside out every time you get randy would really suck, even if your prince is down with it.
You know, on second thought, I'm not sure this would be a fantasy rom-com. The more I read the premise above, the more it sounds like porn. Weird porn. Weird fantasy genderswapping porn. That said, I'm not sure that would be a bad thing...