As far as political correctness goes, the term "birthmother" was created sometime in the early 1980s by the adoption industry in an effort to appease adoptive parents' objections to the term "natural mother". The term "natural mother" was in use for a long, long time before "birthmother", not vice versa...
When I surrendered my son for adoption in 1972, I was referred to as his "natural mother" in the legal relinquishment papers I signed for the County of San Diego's Department of Social Services. (It's right there in black and white on my copy!) "Natural mother" was also the term that every social worker from that era used.
I don't think that "natural mother" has anything to do with society considering the adoptive mother to be "unnatural". An analogy would be this: Many women choose to deliver their babies using some form of pain medication or local anesthesia. Does this mean that they've undergone "unnatural childbirth" just because they opted not to go for "natural"? The term "natural mother" is merely used to denote the "nature" or biological aspect of birth.
When the county adoption agency contacted me in the middle-1980s to request I update my medical history, they addressed me for the first time as DS's "birthmother". And I was totally shocked! It felt like I was considered to be some type of mechanical incubator or something. It just didn't feel right. But I played along with the change in terminology with the agency, adoptive parents and DS, himself. God knows, I didn't want to offend anybody...I didn't want to do anything that would lessen my chance of reunion. I do still use the term mainly here on the forums, but I'm not fond of it. It just reminds me too much of
The Handmaid's Tale or something.
The one terminology change I never accepted or adapted to is "birthson". DS is not my birthson, he is my son. I may not be his mother in the world's eyes, and that's fine. But one thing I know in the deepest part of of my soul is that HE is my son.
PS: Please don't take offense at your son's bmom for asking you to use the term "natural mother". She just wants to feel better about herself, IMHO.