Maybe it's this bit-o-scripture concerning the gender of bishops....
1Ti 3:2 A bishop then must be blameless, the husband of one wife, vigilant, sober, of good behaviour, given to hospitality, apt to teach;
Of course same sex marriages could possibly be a workaround in this instance with two women in a marriage with one the 'husband' and the other the 'wife'. I guess then that a bishop could be a female and the 'husband' of one 'wife'.