So you are comparing trans women on estrogen and low testosterone playing sports to child marriage. Nice.
Right -- in both cases, it is unfair.
Uh, no. The first is unfair, the second should be a criminal offense.
Marriage of anyone under 18 is legal in most US states with parental permission. The marriages often involve pressure by parents on young women for cultural, religious, or immigration-based reasons. Thus, unfair to the young women. I'm not seeing it as a criminal offense unless trafficking was involved. YMMV.
Nuances matter. I support marriage in general, but not without restrictions.