In the former case the doctors conflicts of interest don't seem so strong, in the latter case the doctors conflict of interest is overwhelmingly self-evident, so there is much higher risk of the doctor consciously working against his patients interests when the patient is still a minor in a major power differential.

One causes sterilisation, one causes pregnancy, western society is specifically very very against 17 year olds being pregnant and are anti-natalist and sex-negative more broadly.