Hi Tupperwarewolf - I didn't think you were having a go, and the extent to which I've posted on this thread is ridiculously disproportionate to the strength of my feelings on the subject! :)
Yes, the treat v necessity aspect comes into it. To use your own example, I would certainly (and gratefully) accept a kidney from another person if my life or quality of life depended on it. I do not, however, find anything particularly appealing about having another person's kidney transplanted into my body, and if someone offered to give me a kidney just for the heck of it, I'd think that was a bit disturbing.
I would also gratefully accept breastmilk if my health depended on it, and I think it's wonderful that women can donate breastmilk to babies other than their own, whose health may depend on it. I do not, however, find anything particularly appealing about consuming another woman's breastmilk, and if someone offered me some (in ice cream form or otherwise) just for the heck of it, I'd find that disturbing as well.
My need for life and health trump my need for appropriate social boundaries by a long shot, but, for me (and I'm really only trying to explain why the idea of breastmilk ice cream is quease-inducing for me ), those boundaries trump pointless experiments like making ice cream out of each others' bodies.
Consuming cow's milk ice cream - which I do - is a different kettle of fish for me, in much the same way as carnivorism has very different social implications than cannibalism.
I'm not saying that consuming human breastmilk as an adult is on anything near the same social-inappropriateness level as cannibalism, just that the issues it raises concerning the differences between inter-species and intra-species bodily product consumption can be understood in a similar way.