Supposing the interaction were strictly platonic, then there’s no hang-ups about seeing other people so long as everyone’s on board with that arrangement. The trouble is that it is very rare for nursing to be strictly platonic for both parties simultaneously. And of course that even if it were, situations change and over time it could get messy.
But more to the point, that if both parties are interested in for instance a tandem-feeding and everyone agrees to a few basic ground rules, then why not? Swinging exists. I will neither condone nor condemn it, but it does exist.