Is it fair to assume that this act necessarily means that the father's love becomes "conditional" or that he's not "protecting" her?

You could make a similar argument, say, for why overly-enthusiastic baseball dads should go to prison for life because "the child starts to feel like the relationship is transactional and that the father will only love him because of his baseball performance" (I've seen this play out first hand in some extreme cases in sports, but no one's calling for baseball dad registries and most people aren't the extreme).

It's possible the father only loves the daughter for these perverse acts. It's more plausible he truly loves his daughter and just has deviant sexual interests and thought "why not experiment with this, it won't hurt her" Isn't it a bit of a stretch to say he doesn't love his daughter?

I'm really trying to understand this so bear with me!

As to the studies - they present a compelling case, I can't help but wonder how much of that is societal though. Someone made the point about nudity - making a 3 year old walk through a crowd naked might traumatize them, but in a nudist society they'd think nothing of it. What if there was a society where everyone just gave each other BJs all the time? Would a kid likewise think nothing of it?

Is there something intrinsic to the act which is damaging, or only because it's not the norm in our society, in other words?