again, this is open to interpretation! Sure a marriage contract is between 2 people and procreation emanates from them. As in egg meets sperm. So just as another woman may breastfeed and nurture their child, another woman may provide a womb to nurture their child. If the couple so decides.
I still can't imagine breastfeeding and surrogacy being same thing. Keeping a child in womb then bearing it is different than just feeding him/her.
The status of one who gives birth is far more greater than the one who feeds on her milk thus the former is not to be left alone without any protection and without her child after she has given the birth like people do in common surrogacy.