1. Risk is a normal part of doing business, especially more so for a startup, and it is logical for a business owner to want to reduce risk. As you say, there are many sources of risks to employees that have a direct effect on the business.
2. The division in this discussion seems to stem around how best to manage the risk of project impact caused by losing a critical team member owing to pregnancy event.
3. Extreme positions:
a. The risk to the business is little different from any other unplanned risk that the business needs to deal with anyway, and to assume otherwise is to be sexist; i.e. there may be women out there for whom pregnancy does not impact their work any more than an illness, and assuming that all women are equally likely to fall pregnant and handle pregnancy in the same way is discriminatory.
b. An ethical/strategic position whereby the additional risk is accepted because the benefits gained in terms of equity makes the risk acceptable.
c. Pregnancy is a flat-out optional potential risk to the business and since a business owner wants to reduce risk then it's logical to avoid employing women.
4. IMO this is all a matter of opinion and has everything to do with one's appetite for risk, one's risk exposure (capital & risk management) and one's principles. The same person given two different startups could come to different conclusions depending on these variables, for instance.
Given this, I really don't see the point of this discussion as it is pretty much subjective and depends entirely on the startup in question and the owners invested therein.