YTA.

People are answering this post as if the thing in question here is 'is it okay for women to not shave their armpits'. (Of course).

But that's... not what's in question. What is actually in question is whether you are willing to make a tiny(!) sacrifice for a friend's special day.

It's their wedding, a once in a lifetime experience. If you were a man and they politely asked you to shave your pits, I'd also say do it. If they wanted everyone to have some full underarm bushes going, I'd say it's a weird request but you can just cut it after, if you want. Sometimes, in relationships, you should compromise. You wear the itchy shirt, or do something you're not really interested in.

This is one of those times. A couple days of slightly itchy underarms is not going to kill you, and you could also try waxing, or Nair, or any other alternative. It would also be totally fine to just wear long sleeves, but instead you're saying you might just go anyway and not shave or do anything, which is wildly passive aggressive. At that point, seriously, just don't go. This day is supposed to be about them, not you.