My own theory of why atheist relationships scare people: personal or political

[Binkie The Clown]

1. Seems to make a lot of sense to me.

Tribal identity, and the need to establish membership in the tribe may go back to basic survival instincts. If you are in good standing with the tribe you will get a share of the hunt, and have assistance building your hut, or grinding your corn, or fighting off the menacing predator. But if you don't fit in, if you make other members of the tribe uncomfortable, or are different from them, this threatens their own sense of tribal identity and the impulse is to force you to conform to tribal norms, or failing that, to banish you from the tribe. Once you are without your support network, the tribe, you face a very real risk of starving or being eaten by a predator. So our brains are hard-wired with the urge to belong and to fit it.

Free-thinking individuals are probably a very recent mutation, made possible by civilization, so in a sense, we atheists are mutant misfits who others, non-mutants, are liable to see as strange, peculiar, "other", etc. I'm glad I live in a society where I can safely be a strange, peculiar mutant misfit.