There is a more symbolic way in which to render this point. Things we have rights to are by this very fact no longer truly gifts. Gifts are simply things to which we don't have rights. Reproductive technologies which seek to 'take' a child apart from sexual intercourse do not treat a child as what he or she truly is. Moreover, recognition of children as gifts underscores the most proper context for receiving that gift.
Sexual intercourse is the mutual giving of partner to partner.
And the idea is that the most proper way to conceive a child, who is a gift from God, is from within a context which is itself a giving one. The very nature of a surrogacy arrangement rules such out. Reproductive organs are purchased by patrons of surrogacy just as sexual organs are purchased by patrons of prostitutes. A related point voices concern about how paid surrogacy renders those women who are already economically marginalized even more vulnerable to coercion and manipulation by those with greater means. Still others compare surrogacy to adoption.