Well, the original argument wasn't just "according to our beliefs, God was involved in this"; it's implying that bananas would be unlikely to be the perfect siz

Well, the original argument wasn't just "according to our beliefs, God was involved in this"; it's implying that bananas would be unlikely to be the perfect size and shape for humans by chance, i.e. unless they were designed for that purpose. Which means there has to be a designer (who cared about humans), which for natural phenomena would have to be God, or at least some sort of god. That is, if you exclude evolution and other natural optimization processes; those are known to be able to simulate the appearance of intelligent design, but in theory could be excluded if there weren't a plausible mechanism for something to have evolved in response to humans, e.g. because they haven't been around for that long in geological terms. (Except that plenty of other animals with opposable fingers have existed for longer, but – details. I'm not saying the argument would actually be valid, even if bananas' current appearance were entirely natural.) On the other hand, artificial breeding provides an explanation that doesn't require God in any way, so it does invalidate the argument. The only way to salvage it is to say that God is responsible for it being possible to breed bananas into such a form, but that's obviously much weaker.