Disposable diapers and cloth diapers do not cause infertility.
Although the diapers can keep the baby's crotch warm it does not lead to fertility in baby's as they do not have the sperm that is fertile like adults would have.
So even if the diapers keep the baby's crotch warm there's no risk in the baby getting infertile as an adult when they wear disposable diapers or cloth diapers.