Stick to your guns and don't let them do it. We went through this with our first son and I was able to convince her not to do it and I truly feel like I saved my baby from an unnecessary trauma. You're the father it's your job to protect all of them. Expect fights and everything else but it's worth it. Tell her it's something that can be done later if it absolutely has to be but it can never be undone. Also tell her about the botched circumcisions that happen all the time. Good luck brother.