This is a very polarised debate.
I think some baby deaths are precipitated by smoking in pregnancy. No point pretending otherwise. I think I have read (it may have been on the Lullaby Trust website and I know they are not necessarily the most impartial of organisations, but they do work with proper figures) that up to two-thirds of SIDS cases may be preventable if nobody smoked during or after pregnancy. Babies of mothers who smoked may succumb to chest infections or similar because of the previous damage to their lungs, where otherwise they might have recovered.
As far as 'dangerous middle-class pursuits like co-sleeping' go - bedsharing (technically not the same as co-sleeping) can be dangerous if done in an unplanned way or when risk factors are present (one of which is, ironically, smoking). Done safely and awarely, there are even suggestions that it may be protective against SIDS. But - much like the alcohol in pregnancy debate - health authorities tend to assume women will be too thick to distinguish between safe and unsafe bedsharing and put out scaremongering messages.
The author of Expecting Better - whom nobody could accuse of having a misogynistic judgy agenda - suggests some alcohol and caffeine, for instance, is fine, but comes out unequivocally against smoking.
I do think we are harder on women than on men, and on pregnant women than on others, for perceived failings, though. There is some legitimacy in the argument that they are responsible for another human being, but I absolutely can understand (and to a degree share) the unease at that because of its potential to help roll back the very fragile gains women have made in terms of being recognised as autonomous beings. And I do think there are classist attitudes mixed up in all this.