Is this a sexualised behaviour?

My 3 year old does things like touching with his penis, wanting to touch another person’s bottom, wanting to cuddle my stomach, etc. I don’t understand where this is coming from because it is not a learned behaviour. He does not attend school yet, I am a stay at home mum whose husband is abroad, we barely go out and when we do, I am always there. He has never had a sleep over. He has never visited family members without me being there, I only allow him to watch cartoons occasionally and supervised, so I don’t understand where this behaviour is coming from! When he does it, I gently but firmly caution him. He starts school in September and I worry if he does something like this in school, how alarming it will be for the teachers who will think something is going on. This is my first son so I don’t even know if this is normal behaviour? How to handle it? Help please.