I mean, it can happen, but it's unlikely to work out long term. I think a lot also depends on how mentally challenged are we talking about?

This might not be the same thing as what you mean...

But Amy Schumer has said something to the effect that she loves that her husband has autism, as basically (with his spectrum) he can't lie, and if he tries, he's absolutely horrible at it... which basically has allowed her to trust him more.

Gray area here. As long as they're both over the age of consent AND she's not in any kind of position where she is responsible legally for his care, like working in the group home he lives in - shouldn't be a problem. Any other questions, call local mental health and talk to Recipient Rights Officer.

I guess it depends on if they can consent. Otherwise it could be predatory.

Possible, but it can be challenging

You have to be mentally challenged to find mentally challenged people attractive