First of all sexual relationships are not "dirty", they are part of the human itself and again, people can feel a genuine desire of having a kid without feeling lust. Your logic literally means the following: Want to have a kid -> Needs to involves sex So because a sexual desire can also be lust -> So that means all kids are product of lust Which is completely a generalization and I don't agree with that.
Of course, the sex is the part of human as you admitted. Having a kid is definitely the product of sex. Imagine a strong undefeatable nearly-immortal guy getting killed just because of his "desire to have family"