I do think it should be illegal. However, the costs of making it actually safe could be prohibitive regardless. There is the issue of diseases. New classes of retroviruses are being discovered in humans. Even with cow's milk there is (now) a supposed danger with Bovine Leukemia Virus. Several new retroviruses have been identified in humans, and certainly these are possible sources of contamination that could be spread.
What happens in ten years when everybody that went to the ice cream shop gets some weird retrovirus? What happens we find out about other unknown effects? Humans stop drinking baby's milk for a reason.
But I think the source of your original post was that I was somehow attempting to legislate morality. First of all governments come with a set of ethics or morals. You can't have a rules without having reasons for them. Its really that simple. Frankly, as a gay person, I think your comparison is erroneous. I hardly think you can equate orientation (which I believe is a characteristic you are born with), to commodity exchange of bodily fluids.
Additionally, I do not believe corporations have freedoms or morals. They operate for the dollar. If women choose to individually sell their milk, sure let them. I think a private market for this could be an important step for mothers (who are willing) to share their milk with women who are unable to breatfeed (medication or disease), or perhaps even medical patients with some strange dietary requirements. Health exams would be necessary though with regulators to oversee it. Any market however would have to directly line the women up with buyers, and I do think that there should be recommendations and procedures in place to protect the health of the women. The funds should be directly transacted with no middle men and there should be health guidelines and limits placed on the amount that they can give. Even dairy farmers have limits and regulations. Even prostitutes have to submit to health exams. Even if you think that legislating such a practice is legislating morality (if not hygiene) there are still health considerations to be taken into account.
What I object to is a corporate adventure that turns women into commodities. There is no "morality" at stake when curtailing a corporation. They simply aren't people. They don't have morals, or rights. They have financial obligations and that is all. Yes I do think it should be illegal for corporations to control this process and to create a product market aimed at "mass' consumption. Corporations have impacts and effects: economic ones on communities and families. Those I believe we have a moral imperative to place legal limitations on. I think prostitution is horrible and has detrimental effects on women. I still believe it is their right to do what they want with their bodies. Having said that, I still think it should be illegal to having a "WalMart" style brothel. You can imagine the consequences.
Moral outrage here is appropriate, even if legislation is not.