Jen "I think it's fun" is easier. But agree - no-one IRL knows the full reasons.
If you give "fun" as reason, people might expect you actually have the key.
For example at a security checkpoint (airport or otherwise) and expect you to take the belt off so they can go about their business as usual and you are not making their job harder.
If you make clear you wear it for a reason (willingly entered agreement that now is binding), they will have to accept that you cannot easily remove it and the have to handle your belt as part of their job.
And by the way: Yes, you can board a flight in a chastity belt. The devices of course detect metal, but it is not metal that is actually prohibited but a weapon.