Are you talking about direct object pronouns? At least in the case of Spanish, lo/la is the pronoun for a masculine or feminine noun. It would obviously follow that it's the pronoun for a man and woman, respectively, the same way they would be the pronoun for any other masculine or feminine noun. I don't see how addressing men and women (as a noun) the same as you would any other noun in the language (save some irregularities) means that the cart is pulling the horse.
It literally means he and she in Latin. Then people started using these to indicate that something was a specific object instead of an object in general.
