"Préservatif" is such a weird euphemism.
Isn't it (less bizarre)? My dictionary gives the medical meaning of prophylactic as "intended to prevent disease"; using it to refer to contraception just requires a more generous conception of 'disease'.
By contrast, if 'préservatif' in French were translated to 'preservative' in English, it would mean, again according to my dictionary, "a substance used to preserve … against decay." I find it a bit more of a stretch to generalise this to pregnancy.
On the other hand, maybe it is just my English-speaker-ness showing through that I can think of pregnancy much more easily as a disease (it literally causes dis-ease …) than as decay.
And while pregnancy may well be viewed as a disease or a cancer, after a fashion, at least condoms protect from a wider range of affliction.
As a native English speaker who has learned French, I’ve often run into “faux amis”. Perhaps the most embarrassing was “préservatifs”, which I expected to mean preservatives but actually meant condoms.