As men don't have the same ability to reach the same productivity as the women would there is no way of comparing them and seeing if they are equal.
If men could breastfeed as easily as women then I can see a basis for an argument, but it is genetically the women that has this ability, and so for the man to do it would be sillyness.
Babies only tend to make an attachment to one person for the first couple of months, they need the person to be constant and the one that looks after them, being the primary care giver. After this they can interact easier and make attachments with others, but this is only after the first relationship has been made. When all men can breastfeed on demand for a baby the argument will be valid.