Yes I think it absolutely is weird, and dubious.
In your first post I think it wasn’t clear that the children were sitting directly behind you so your son only had to lean forwards to talk in your ear vs walk down the aisle. But I’d have asked for clarification rather than started grumbling at you personally!
Anyway, you cleared that up fine so we all know exactly what happened... and what happened was really odd.
What did you do at the time? Did you realise or were you turned away? It wouldn’t be something you’d be expecting after all.
I’d be thinking that the man wanted to be sat on I’m afraid, unless there was an obvious other explanation? I’m hoping there was an other explanation.
However I fundamentally disagree that that it couldn’t be something unsavoury as no one would do that ‘right under a parents nose’. This kind of thing happens exactly there I’m afraid. In plain sight and they get away with it because no one believes it. So don’t rule out this explanation just because ‘it’s unlikely’.
If you do think it was this kind of thing, it may be worth ringing the transport police to see if they think it’s worth looking at cctv to identity if it’s a repeat offender.
It may also be worth talking to your children about what to do in these types of circumstances, if someone acts inappropriately in a public place. Say no loudly! Call it out or question it ‘what are you doing?!’ and tell an adult. Don’t be silenced by ‘having to be polite’. And safety in numbers, so don’t leave friends or get off at a lonely stop if they feel at all worried still.