Whether we want to admit it or not, we sometimes judge people on the way they look. If we see someone who doesn't fit our idea of what being beautiful is with someone who we perceive as "way hotter," it piques our interest because we want to know why that person is with them.
He/she must be rich.
One of them got blackmailed.
He knocked her up after a one night stand.
He/she's secretly gay and in denial.
We're willing to throw out harmful theories we say to ourselves or our close friends who vocalize equally horrible thoughts. But at the end of the day, who the heck cares if the two people are truly happy?