Could you imagine saying to someone, oh sorry, I can't hang out with you anymore. Your IQ is TOO low for me. Lol. How silly.
A low IQ causes people to not think before they act, and to not take in the big picture of things before making very offensive judgments and preconceived notions, and offend people with them.
And the selfish idea that you get to voice your opinion about your partner's family, without thinking about how that makes your partner feel, and without thinking that the person you don't like is MORE important to her, than you are.
That's how a low IQ can ruin a relationship.