Well in a complicated world there can be two factors that lead largely the same result.    This pains some people who have trouble recruiting the energy to think through the dynamics and differences of each.   They want to treat everything the same.   The call to the priesthood is different from the call to brothers or to sisters.   Different vocations and demands and sacrifices.

Both brothers and sisters vs priests are dying out for some similar and some different reasons. 

Opus Dei has unmarried members, and also priests.   All of their priests were once unmarried members of Opus Dei.   They all held "real" jobs (engineers, scientists, lawyers, finance folks) before they became priests.   

The unmarried members don't wear habits either...but there is the celibacy requirement, and yet there's not a worldwide drying up in their vocations either.   Hmmm.   Additional thinking and hypotheses required.