Conservatives might not have a logical argument against this, but feminists have plenty. And if you read radical feminist writings they've been warning about this and the outcomes of this for decades. Take prisons for example. The policy changes that in some states now separate prisons by "gender identity" instead of sex has led directly to males being locked up in women's prisons, some of whom have sexually assaulted, raped and impregnated female prisoners incarcerated with them. This is just one negative outcome of many. The ideology behind "trans" causes demonstrable harm towards women and girls.