Probably biological. Catholic and Eastern Orthodox (iirc) are the ones that stop clergy from marriage. Other ones allow for that and kids. What can you say Christianity are all a bunch of Sects fiends.
"Mother"/"Mother Superior" -is- a valid way of addressing a 'head' nun, though it is rare to use it nowadays. They usually prefer "Sister [insert name]" now.
Source: Worked part-time in a Catholic diocese office for awhile.