truly healthy babies

so there have been tons of posts related to "what would you want the sex of your baby to be?" and the options are usually 1. boy 2. girl 3. healthy. normally, the majority votes for healthy, no question right? at the end of the day, all that's truly important is a happy, healthy baby...right? but when you read posts regarding transgender children, teens, and adults, it made me realize that a lot of the same women who stand for happy, healthy babies can be highly transphobic. i guess my question is: after 9 months, hours in labor, and years of watching your child grow up, if you truly only cared about a happy, healthy baby, what are some valid reasons to be unwilling to accept them as transgender?