I had this discussion with a group of friends when dd was tiny. We were all bf'ing as we chatted. We all said we wouldn't mind feeding someone else's baby if necessary. No problem. Then someone phrased it the other way around - how would we feel about someone else feeding our baby? All of clutched our babies closer to us and said "no way!".
I clicked on this thread to find out what "cross nursing" was. Had a vision of a bunch of hairy blokes in womens clothing pretending to breastfeed.