I would suggest that Airbnb ask the parents of these babies if they are, in fact, people. My guess is that most of them ;) would say that yes, they really are people too. If the parents consider them people, then I think the company should believe them.
Also, they might want to ask the parents if these tiny little adorable people are also "work". Again, my bet is that most parents would agree that there is additional work involved where babies are concerned. This is not limited to the parents when they are guests at someone else's home. There is additional water (babies take baths! and use sheets!), and loads of laundry involved. Additional space is needed - babies require beds. Most people don't have cribs at the ready for any incoming babies, and would need to prepare (and might not even have the space. They frequently throw up, or the diapers often don't do their job of containment 100%, causing additional attrition of sheets, slipcovers, etc.
There is also additional liability involved where there are decks, pools, hot tubs..
I speak as both a parent, and someone who genuinely likes being around babies, and thinks they are charming. However, I understand that not everyone thinks so. People should have the right to not have children in their homes if they don't want to. If people don't want to take on the risk, prep work and clean up, that should be entirely up to them. Some people are renting out small bedrooms for two people in small apartments. If I were living in one room in a small apartment, renting out the other, I would not appreciate a couple bringing up to five infants into that space. I also believe it would be very unsafe for the babies, and a huge liability for the host.
Last comment - Five babies is not five times the work and mess. The differential is not linear. it is exponential.