This is actually the first line to a joke, with the answer to the joke being “they like to fascinate.” This is a play on words because it sounds like the phrase “they like to fasten eight.”
A phrase that sounds the same as another phrase when spoken aloud is called an “oronym.” This is similar to a homophone, but a homophone uses just one word. An example of homophones is “allowed” and “aloud.” Of course, “fasten eight” and “fascinate” are examples of oronyms. Other examples include “I stink” and “iced ink" and “the good can decay many ways” and “the good candy came anyways.”
Jokes like this one often rely on playing around with words, since humor comes many times from surprising listeners in some way. This particular joke can mean either that girls like guys with eight buttons because they like to “fasten eight” of them since it is often fun to dress members of the opposite sex for lengthy amounts of time, or it can mean just a generic statement about how girls like fascinate guys with strange preferences in shirts. It is the unexpected nature of the double meaning in the oronym that is the punchline in this particular joke.