Hi thoughtfulone,
Sorry for the delayed reply, I’m not able to keep up with the pace of discussion here!
I mean, wouldn’t it be against natural law to let a doctor cut off a piece of your face with a knife as in rhinoplasty, or to cut off your nipples and cram a pouch full of silicone into your chest?
Unless there was something defective with your nose, then yes I believe this would be against the natural law. God chose for you to have the nose you have, and spending a considerable amount of money to alter it would be a violation of his Will I believe. However, the sin being committed here (if freely with full knowledge) would be vanity and self-love and not necessarily mortal.
Is everything that goes against Natural Law a mortal sin?
No, first the matter has to be grave, with full consent and knowledge. Not all actions contrary to natural law would be grave. However, my understanding is that ABC is a grave matter because it interferes with one of the greatest gifts God gives to mankind, the ability to co-operate in bringing into being of a new human being with an immortal soul. That is essentially an infinite gift, and any obstruction with God’s will in this area is clearly very serious.
I always found the Natural Law argument to be tenuous, but I should probably read more about it before discounting it altogether.
It is a massive topic, with countless volumes written on it, and I have only just scratched the surface. I would recommend Edward Feser’s books for a good introduction to the concept.
With regards to your breastfeeding point, I would agree that not breastfeeding is potentially against the natural law - I believe Pope John Paul II spoke of it’s moral goodness. However, whether not breastfeeding is necessarily a sin is another matter. If formula feeding is undertaken due to sheer laziness, I would probably think it would be sin, though not necessarily a grave one, as the more fundamental natural law of feeding your infant is still met. However, if there are severe problems with the breastfeeding process causing distress to both mother and infant, then there is some deficiency here (either physical or mental/emotional) and therefore alternatives can probably be sought if available and
almost equivalent. I don’t believe this is necessarily a sin.
Anyway, those are my thoughts, I hope they are of some help. My prayers are with you.