It’s whatever the parents want to be called. If they want to be called by their first names, then so be it. I don’t understand how it can be disrespectful if the parents don’t mind it. Only the person being disrespected can determine if something is “disrespectful” to them. Nothing is universally disrespectful in every single case in every single possible arrangement. It’s disrespectful to call anyone by a name they don’t want to be called.
I do happen to think it’s weird only because it makes it seem as if parents are on the same level as anyone else, even though they are something special and calling them “mom” and “dad” differentiates from that. Also, I’ve always called my parents “mom” and “dad” or “mommy” and “daddy” when I was little, which I still do every now and then. :)
But I have a friend who calls her dad “dad” and her mom “Kiki”, even though they’re both biological parents. It’s just what she does and her mom has no problem with it. She doesn’t find it “disrespectful”. I’m not one to criticize their family situation and what they choose to do. It’s none of my business. What works for them is fine.