[kx9088]

I'm not sure I follow you.

[ninenot]

It's ok to alter nature when it helps to save lives, but it's not ok to alter nature to prevent unwanted pregnancies

"Save...lives" is a POSITIVE phrasing and your instinctive use of the term contrasts with "prevent...pregnancies," a negative phrasing.

Of course, preservation of life is a positive--foiling the creation of life is a negative.

Your instinctual use of the language is precisely correct.