I don't mean to be dense, but why do you think that's so?
The classes aren't homogeneous; that's the point of defining classes. Why should they suddenly adopt similar behaviours when a relatively small thing, access to birth control, is changed?
m.
,--------------------------------------------------------------------------.
> | The most stupid ideas can now in a <
> Marco Anglesio | moment be transferred into a thousand <
> mpa at the-wire.com | volumes and spread abroad. <
> http://www.the-wire.com/~mpa | (Angelo Poliziano on the invention <
> | of printing, 15th Century) <
`--------------------------------------------------------------------------'