1. give to your girlfriend tell her,if u give that Private key to someone, we will be finished. ( never do that btw.)
She's still your girlfriend and not yet your wife so better not to give details like this unless you really trust her. I don't have trust issues with girlfriends or what but as long as you haven't tied the knot, you should keep it on your own.
It's the same way of your explanation about giving your PK to someone.
There's a possibility that your gf will leave you after taking the funds on your PK.
totally agreee, why not giving it to your sisters or brother instead of girlfriend.
But for me its better to print the private key add frame and put it in your room that way it will not look like a private key . those people who can only notice this one is the one who has background in crypto.