Why do they call testicles family jewels?

When families kept their wealth in the form of jewels (there were no banks), loss of the family jewels could spell the ruin of the family.

If man's testicles have been damaged so that he cannot have children, his family is also ended as he cannot participate in extending it.

So the term family jewels came to be a euphemism for testicles in polite society where even mention of a "leg" rather than a "limb" was considered to be vulgar.
When families kept their wealth in the form of jewels (there were no banks), loss of the family jewels could spell the ruin of the family.

If man's testicles have been damaged so that he cannot have children, his family is also ended as he cannot participate in extending it.

So the term family jewels came to be a euphemism for testicles in polite society where even mention of a "leg" rather than a "limb" was considered to be vulgar.