Saying 'no' to making it more socially acceptable for a man to be a stay-at-home dad, and then going on about how 'not everyone can do X' seems to make the following statement indirectly:

  Only men are capable of (starting up a company | working at a senior
  level | working at a technical level). There are plenty of people who
  could never achieve that level of competence (namely women), they
  can stay at home.

Unless the poster is making that (very sexist) statement, then I fail to see how he/she is in disagreement with the poster they were responding to. Saying that it shouldn't be socially acceptable for people with great skills to let them languish by becoming a stay-at-home dad is not the same as saying that it should not be socially acceptable at all for a man to be a stay-at-home dad. [ Though in general, the idea that society should pressure people to make decisions that are only in society's best short-term interests. Who's to say that someone working at the 'senior level' is making a larger contribution to society then someone that stays at home to raise the next generation? ]