What's the word for things naturally not attracting or mixing with each other?

How about immiscibility? This extends the analogy from chemistry of mixing, that you cited regarding oil and water. But your suggested sentence might read a little better, if it relaxed the single-word requirement and was something like:-

I don't find them interesting and the feeling is mutual. So I'm happy for us to be socially less than miscible.

Ref miscible (in Chemistry)

Here, 'Repulsion' can be used. But it's a strong word. In simpler cases you can go with the words 'Aversion', 'Disinclination'.

"I don't find them interesting and the feeling is mutual. So I'm happy for this natural disinclination/aversion"

segregation

For example from the Journal of Psychology :

Natural segregation occurs everywhere in nature, in business (all auto agencies in same neighborhood). Natural segregation allows like to enjoy like.

And from Sociology:

Natural segregation is found wherever cliques form.

Dissociation is a state of being separate ("not mixing") without the need to be repulsive.

I don't find them interesting and the feeling is mutual. So I'm happy for this natural dissociation.

ODO:

dissociation NOUN
1 The disconnection or separation of something from something else or the state of being disconnected.

‘the dissociation between the executive and the judiciary is the legacy of the Act of Settlement’

‘What Shiang herself experiences in Paris, as an almost accidental tourist who doesn't speak French, is severe dislocation and even dissociation.’