What is unreasonable: being blind to the fact that some women find pregnancy akin to an illness, ie, they feel sore, ill, tired, need medical intervention, need rest and can not and should not run marathons.
What is reasonable: expecting all women to maintain a decent level of fitness, whether by walking, running, cleaning the stairs, whatever. Its not fair or acceptable to allow your health to decline in preventable ways when you're pregnant/have children - you need to look after yourself to care for them.
Yes, yes, i recognise there are some things that can't be avoided, but a hell of a lot of pregnancy related complaints, can be controlled through diet and lifestyle (before and during pregnancy). If you just think fuck it, I'll eat pizza every night and get the bus everywhere instead of walking don't be surprised if you get more aches and pains, gain more weight, have higher risk of GD, SPD.
To the PP who suggested midwives run exercise classes, what a brilliant idea. That would make the concept of exercising while pregnant mainstream.