Why are some people turned on by the thought of their significant other cheating on them?

You don't say that you're turned on by the thought of your significant other cheating on you, so I'll direct my answer to the "some people" that you speak of. These folks, who are not you, are lucky because there is a wealth of fascinating theories on the topic.

This particular kink is called cuckoldery, courtesy of female cuckoo birds known for laying eggs in other birds' nests. Initially, the term "cuckold" was used to describe men with adulterous wives, particularly those who were duped into raising other men's offspring – a timeless, deep-seated fear that has been immortalized in the work of literary greats like Shakespeare and D.H. Lawrence. The meaning has since evolved to include those who are turned on by their partner's infidelity. Fast-forwarding to the Internet age, you have "cucks" who actually advertise for and seek out partners -- sometimes referred to as "bulls" in this sexual subculture -- for their wives. Believe it or not, this genre of porn is currently "the second most popular heterosexual interest in English-language search engines," according to the bible on such things, Sai Gadaam and Ogi Ogas' "A Billion...