Obvs she can't make me as I am her manager, but I have created unpleasantness.
You didn't create any unpleasantness, OP.
You feel bad because women are conditioned to be kind and comply with societal expectations and because it's natural to empathise with the less fortunate.
This pronoun initiative is pitched as a way for people who don't care about pronouns to signify solidarity and support for people who do, on the basis that people who care about pronouns are less fortunate and therefore more deserving of accommodation to others' detriment.
Inclusivity initiatives such as this often weaponise misogyny to railroad women into feeling guilty for having boundaries. Most women don't even notice it so they get caught up in complying as that's just what you should do to be kind.
Don't feel bad at all for having boundaries, OP - you're just as entitled to boundaries as anyone else, and it's worth reminding yourself every so often that you don't want to do this and you shouldn't be forced to do something you're uncomfortable with against your will.
As others have said - you don't need to explain your decision. Saying you don't feel comfortable with it should be enough. Women shouldn't be expected to require a vast knowledge of a practice before they're entitled to say no to it, so don't feel guilty or wrong for not being able to articulate why your answer is no - the 'no' part is the important bit and they should be respectful of that.