Justifying one thing because it's a necessary component of another *unnecessary* thing... what logical fallacy is that?

A pregnancy which we force upon them, sexually violating them, yes. But that doesn't mean they don't care for their children. They want to nurture and protect them, and naturally develop a maternal bond. Biologically the milk is made for their calves to drink, and allowing them to, not stealing them away and killing them, is in the best interests of both parties involved (the cow and the calf).