Let me express it in formal logic for you.

Your argument is that:

If you do not {~} have a genetic investment in a child {P}, you will not {~}form an attachment {Q} to them [hence, pedophiles will not form a lasting attachment to children].

This can be expressed as:

~P > ~Q

{If P {having a genetic investment in a child} is not true, then Q {forming an attachment to that child} will not be true}.

It follows that:

An adoptive parent does not have a genetic investment in their child {~P}.

~P, (~P > ~Q) |- ~Q

Therefore (|-), an adoptive parent will not form an attachment to their adopted child.

Since we know that adoptive parents do, in fact, form attachments to their children {Q is true}, then we have the following:

Q, ~P, (~P > ~Q)

Which leads to:

Q, ~P, (~P > ~Q), |- (Q ^{and} ~Q)

Since Q ^ ~Q is a contradiction, we have reduced your original premise to an absurdity (Q cannot be true and not true). Therefore, by reductio ad absurdum, your premise is not true.

~(~P > ~Q)

QED.