Because she, like you, doesn't believe you should be given full control over something she will have the responsibility for. This is her way of protecting herself.
You know exactly why. Nice way to avoid your own logic though.
Finances are completely different in that a person controls their own body. Money, no matter if you are employee or an owner or investor, always comes from and is controlled by somebody else but yourself.
If something impedes upon your money, do you have the right to kill it? No. If someone impedes upon your body/life, in many cases, you reserve the right to harm or kill them.
If you kill someone who has robbed you but they are running away and present you no bodily harm, you're going for murder. If this same person is causing you bodily harm, you're generally not gonna be punished for defending yourself to the ends of your abilities.
So, I ask you, are you too getting a vasectomy at the request of the woman you are with If she asks?