OK. In the episode where Ashley and Porky are working on their History project if you look at Porky when Mary-Kate and Ashley are trying to see if he asked Mary-Kate out on a date; you can see that he's trying really hard to keep a straight face and that he almost laughs. I thought it was funny.
Nice catch, but I noticed it too! I think that he was trying not to laugh so as not to upset the girls because he thought it was ridiculous! He even said "That's crazy! Why would I ask out Mary-Kate when I like Ashley?!?" He was just thinking how crazy the thought was. It's called acting.