The gift of life in the womb is just that a gift from God and as women we are called to submit to God (and our husbands) not to seek to assert our will.
The use of NFP, may not alter the actual act, but it does follow from a wish to exert control, rather than submit. Of course God can bless us despite our wishes and intention, but it must be fundamentally sinful to take any action, other than to submit to him, to be open to life and to accept the blessing of motherhood as often as he chooses.
In my view no Godly husband should expect, encourage or permit his wife to do anything except submit to God.