"He must be a slacker.."

[Jocelyn]

It sounds like the OP is describing my nephew and his family.
He and his wife decided the children needed one parent at home, and they were willing to make the financial sacrifices to provide that for their children. As they point out, by the time you pay for good-quality day care, you're pretty much using up one person's salary just on work-related expenses. His wife is ambitious, and would not be happy without a career; he's very here-and-now oriented, which is just great for a couple of toddlers. And he's raising two daughters who are going to be fantastically confident and assertive, because they're growing up with parents who adore them and cherish the time they spend with them. Raising kids who are mentally healthy is certainly a service to society, at least as much as 'you want extra butter on that?' (My nephew managed a movie theater)

I'd be tempted to reply, 'Well, having a parent stay home with the children has certainly been a time-proven method of child-rearing. They think it's important for their family.' And if the other person tried to continue, I'd say, 'Are you saying that parents who stay home are just sponging off their partners at work?' Most people don't say a SAHM is a parasite who should get a job, but a SAHF is somehow unmanly.

[baritone108]

 My how times have changed.  When I had my oldest daughter (now in her uper 20s) DH's and my situation was very similar.  What we heard was how wonderful he was for doing this.  I, on the other hand, was made to feel like a slacker for foisting off the childcare/parenting on my poor DH.  NO ONE taked about how wonderful i was for going out to work and supporting the family 

 In the case at hand, I think I'd just stare at the offending person in silence with a puzzled look on my face, as if I couldn't figure out what in the heck they were talking about.

[Shea]

I think it's a great idea to point out the sexism in their assumptions. It may help them realize how ridiculous their comment was, especially if they just weren't thinking.

[Sterling]

I have to agree with Raintree.  I hate this line of thinking.

[Itza]

You know what? I'd label it and tell them they're being sexist!

The set up works for them so why not?

[Dindrane]

I am sorely, sorely tempted to agree with the idea that you should call people out because of this completely sexist attitude.  But I'm not sure that's going to be the best thing to do in every circumstance.

The thing is, the point of the "interesting assumption" line is to shut down a line of conversation, because it's inappropriate.  So as much as I agree with everyone who says that this is a horribly sexist attitude (and it so is), telling anyone that, or even hinting at it, is going to extend the conversation.

So if you're okay having the conversation turn to gender roles in society or wherever else it may lead, then I think it's fine to say, "Hmm, that's an interesting assumption.  I bet people wouldn't make the same one if it were Katie at home with the baby!"

However, if you just want to stop talking about it, I think a much better way to respond is, "Katie and Sean are doing what works best for their family, same as anyone else."

[holly firestorm]

You know what? I'd label it and tell them they're being sexist!

The set up works for them so why not?

The attitude is also very obsolete.  If they are otherwise intelligent life forms, you could engage in a little gentle education.  If not, you could tease a little.  Something like, "Nah, he's no slacker.  Tsk, tsk, ever since we gave them the vote these broads think THEY should be the breadwinners and make their guys stay at home. He should just club her over the head like the good old days and make that b**** stay at home where she belongs.  By the way, where'd you park the model T?"   Reverse the whole thing and stand it on its head.   You'll probably get the person laughing and they might even realize how silly their assumption was.