Very very true... modern women have the choice to have or not to have children and it should always be the woman's choice no matter what situaiton she is in. I do not think any Dom on here would consider making this demand on a sub, not even those who are in long term relationships with those subs.
As has been pointed out, there are so many reasons why this should not happen. Pick one and use it if you need an excuse (you shouldn't need one but it is still useful to have). If you really have to, get your Doctor to write you a letter saying why he thinks you should not have any more children. Most will be happy to do so.