Bridget Crawford on the well-worn (and worn-out) expression. Offensive for many reasons, the one Prof. Crawford focuses on is how it treats women like a commodity. My girlfriend would rightfully slap me if I called her a cow, and do not conceive our relationship, or any relationship I might have, progressing to the point where I would "buy" her (and if I ever did start thinking along those lines, I'd hope she'd break up with me immediately).
But beyond that, "why buy the cow when the milk is free" acts as if sex is the only thing a woman has to offer a man. While I certainly think sexuality is an important part of a relationship, at least for me there are many other wonderful things that come from being in a relationship. Companionship. Emotional support. Fidelity. I could go on. It's unbelievably objectifying to act as if that one attribute is the only thing of value a woman has that a man might want. And, I might add, it displays a pretty degrading view of men, to think we're that shallow. It's amazing how my gender loves to revel in how backward we can be (anyone who's seen a beer commercial knows what I'm talking about).
Women are not cows. Nor are they talking sex toys. They're real people.