Because you touch yourself at night or BYTYAT (for the Newgrounds BBS (Bulletin Board System)) is a pretty simple and exploitable meme that started on Family Guy episode 27 (or the 20th episode of season 2) called Wasted Talent, first aired July 25, 2000.

As flashbacks are commonly used in the show, it's generally to include a random joke normally not related at all with the story. For this meme, the text goes:

Peter (to Lois): "I haven't felt this crappy since the time I went to that museum"

Flashback

Peter (as a child): "Why did all the dinosaurs die out?"

Curator: "Because you touch yourself at night."

This answer, totally random, then became popular as an answer to everything stupid, boring, unknown, etc. as "because you're gay" used to be in the past.

Why do you drink so much?
Because you touch yourself at night

Why do fishes swim?
Because you touch yourself at night

Why can't pumpkins be pink?
Because you touch yourself at night?

Watch the full episode by clicking the play button on the video below.

Song made using this epic catchphrase

Epic Fail trying to insult a young boy

For a few examples of common use of this expression, check the Wikitionary

You can also check the Newground's Wikia for a proof of its importance on Newgrounds.

This is also featured on the famous Urban Dictionary

The Meme also have a few facebook groupsits
First one here
Second one here
Third one here

And for YTMND, check the up right corner for the YTMND and Wikipedia link.

Finally, there's also "Because you touch yourself at night" pins sold on the internet. But because I don't want to do any publicity for products, I won't include a link. Just check by yourself.

For youtube users, the search for the exact sentence "because you touch yourself at night" links directly to 36 videos and is used in thousands of comments, including comments used for the show of Ray William Johnson