The elders' manual says the "strong circumstantial evidence, such as evidence that the accused spent all night in the house of someone of the opposite sex who is not his wife or a family member" (paraphrase) can be used as evidence in a judicial case.
This just shows how antiquated the Watchtower's thinking is. I have spent the night at single women's homes a few times without having sex, yet I could be disfellowshipped on this kind of "proof." In fact, I've even slept in the same bed with women without it ending up in sex.
Their whole thinking assumes that people are sex-craved maniacs waiting for the opportunity to act out on their secret lascivious desires. As a JW man you're not allowed to be alone with a woman for fear of it leading to sex.
After leaving the JWs, I was really surprised to find out that the real world doesn't operate this way. A good friend of mine who lived with his girlfriend had a male roommate living with them as well. It's now common to have male/female roommate situations that are completely platonic, yet the WT feels that a male sleeping in the same house as a female who's not related to him is strong enough circumstantial evidence to "prove" fornication. They're living in the 1920s. Young people today would laugh at the idea that a single man and a single woman spending the night under the same roof is evidence of them having sex. That's just not reality.