It's not a parasitic relationship! It's a human life. If you saw someone starving on the street and you knew that you could voluntarily give them food or let them die, there is a moral obligation for you to feed them because they're a human being with inherent value. It goes beyond simple volunteerism. Yes you could potentially let that person starve (you could potentially also murder them to stop you from having the obligation to feed them, which is essentially abortion) Both options make you immoral.

Alternatively if I agree with you then our entire society should be based around volunteerism and no one has a right to collect taxation if it is going to be redistributed to another person without their consent.