When would be the moment in time to tell them though? If you never tell them and you've known each other for a while, they might think you can't trust them. If you tell them quite early on (or even late into the relationship), they might freak out and leave.

And, in regards to the freaking out and leaving thing, I really wouldn't blame them. Sometimes, it's simply not a case of how much they love you. They simply may not be able to cope.