Not having sex is still another way to avoid having children. If supposedly using birth control is keeping God from fertilizing us then avoiding the whole marriage 'rights'is not only keeping God from giving us with children it is also denying our partner a sexual relationship...we're not suppose to do that.
If consent is to be the buzz word in all matters sexual, then mutual consent not to have sex does not end a sexual relationship:it just makes a radical change. It's a hard choice, sort of like accepting a Down's syndrome child rather than snuffing its life out. You make that sacrifice. Again, I do not expect anyone to be heroic, and on a daily basis, most of us fail to do even our duties on small matters much less make great, life-altering sacrifices.