Let's Play Devils Advocate...

[Tubular Monkey]

At 17 he discovered masturbation. At 31 he discovered sarcasm. He should discover hygiene at about 45.

 

[HealthyMcWrap]

You know, instead of being a douche, Chris should use one. Would work wonders.

 

[Lady Houligan]

To those who think he's genuinely believing her: despondence does wonders for one's ability to use sarcasm.

I voted douche.

 

[Globe]

I voted that he was being serious, mostly because of this:

Chris: Stuff... candy... don't... I stuff vegetables down my throat! I stuff green beans! I stuff broccoli! I stuff corn! I stuff carrots! …

Because of this I really can't imagine Chris understanding sarcasm much less use it.

Chris probably responded that way just because he was frazzled from how the conversation was going and wasn't actually listening to what Matthew was saying. Kinda the same thing with the whole "I EVEN MOWED THE LAWN!" reply.

If you think Chris has trouble understanding sarcasm, consider this video;

[youtube]w8LZmZRmzQM[/youtube]

He isn't being explicitly sarcastic here, but he is mocking Clyde by way of displacement. If he can do that, he can probably get sarcasm.