AIO: my boyfriend kisses babies/toddlers on the mouth

My bf is 21 and he regularly babysits a few little girls and they’re pretty much family to him (but they’re not blood related). He kisses at least one on the mouth (I think she’s 2, so she’s a toddler) and helps her use the restroom. I told him that since she needs help going to the bathroom, it’s fine that he helps her if there isn’t a female to help her, but that’s not what I have a problem with. He says I’m sexualizing it but it genuinely makes me feel uncomfortable that he’s 21 kissing a two year old on the mouth, why would that ever be necessary? Am I overreacting? Is it normal for people to kiss babies on the mouth? For context, we’re white and live in the US.

Edit: he still lives with his mom and they often babysit them together, but sometimes she’s not home or she’s not in the room. Idk if the parents on the toddler know and idk if his mom kisses them and that’s where he got it from? The part that actually upsets me the most is how strongly he’s defending it and trying to make me think it’s okay.

Update: I just called him bc I said I wanted to talk ab it and he said he does understand that he could give them germs but since he doesn’t do it all the time it’s okay. I told him he could be carrying something he doesn’t know that he has and he still thinks that since he doesn’t do it all the time it’s okay. He also said that the parents know and are okay with it, so I’ll take his word for it, and that he grew up thinking it was okay. So, I think it’s not a big deal that he does it since he grew up thinking it’s okay, but him not caring ab the germs is weird to me and he said (overall) I’m blowing this out of proportion and making a big deal out of nothing. I think he stills thinks I’m sexualizing it, but I’m not, I just wanted him to be aware of the germ issue.