> Your body isn't designed to do anything.

Yes it is. Without procreation species can't exist, so it must be designed to procreate. The process takes quite a lot of energy which is precious to all forms of life and there are all sorts of mechanisms in place to make you wanna do that. Design.

> The fact that you find having children fulfilling is about you, not about anybody else.

Everything is first about you. Its illusion that some things look like they aren't on frontend.

Viruses are designed to infect, multiply and spread. Also humans are not viruses. I know that isn't what you meant since you specified "in the sense that...". But this kind of analogy seems to me to lead to bad thinking.

Viruses aren't designed to do anything either.

Sometimes they are. (But that's not what I mean originally. You seem to object to the word design?)