That is just as much a statement of religious belief as is any dogma of Judaism, Christianity, or another religion.
It is not obvious from nature: seeing my 5-month-old daughter, who has received everything from me except some DNA and air, natural logic could conclude that I have ownership rights over her body, which is, so to speak, me. Your belief is not a general case from historic experience, in which many humans have legally been property of others.
Judaism and its descendant, Christianity, teach us that our Creator ultimately has ownership of our bodies, and that He can make legitimate demands, both in positive action and in restraint from action, on our use of our bodies.
Except that the child is a human individual, whereas in nature, might is right - be it a lion's ability to grasp and tear apart antelope, or a bacterium's ability to wreak havoc inside the lion's body. The recognition of this point is necessary because it is a human societal demarcation - distinguishing the progeny from the progenitor - and is a basis for establishing and recognising the rights of that individual.
There is no law arising from non-human sources which prevents an individual from being another individual's physical property - a slave. In fact, the contrary if one subscribes to the popular religions. Recognising the individual as a distinct entity is the first step in creating a societal system where the individual has the protection against his or her physical self being forcibly used for the purposes of another - being enslaved by a more powerful agent - be it by the parents, or any one else. That recognition of the individual's right to his or her self therefore forms the core of recognising that individual's other rights.
If you choose to fail to recognise this aspect of a person's right to his or her physical integrity, then who sets the boundaries for what is permitted and what is not? One may choose to mutilate male or female genitalia of another individual, while another may decide to have surgery performed on that individual to remove certain teeth or other body parts before they can cause problems at a later stage. Others may choose to do this or other barbaric procedures out of cultural whims. Where does the boundary lie, where the individual has protection from such procedures done to him or her, against his or her will?
This may all sound like a stretch, but they are logically connected. What indisputable argument allows you to lop off another's genitalia without his or her consent, that prevents someone else from doing something similar to another individual's other body parts? Your personal, subjective beliefs is not a valid answer.