This one is kind of a pain to put into English, so here's a basic rundown why:
For those who don't speak Japanese, there's a triple pun here based around what Yuyuko's saying. The key phrase is "onaka ga suiteru", which usually means "I'm hungry" or literally "my stomach is empty". But, "onaka" can also mean "abdomen/insides" (especially if the kanji by itself), and then of course there's "naka", which in sexual situations usually goes with "nakadashi", or "cumming inside". Add all that to the fact that Yuyuko is portrayed as a glutton and the joke is complete, but to put it in English in a simple sentence... I don't think I did it justice. If someone can find a way to make it work, by all means go for it.
