The wife quickly realised what had happened when she returned and her phone had been moved. She confronted her mother about it but the woman refused to delete the photos, not understanding why she was so angry.
"My wife and I have a few weird fetishes and so some of the photos I took that are for her eyes only are really humiliating if seen by people who aren't my wife," the man wrote.
"My wife is really on my side and supporting me but MiL sent her a message asking why she's cross about it. It's just a picture. I'm so mad.
"We're supposed to have her over for dinner this weekend because my wife's brother and his wife are in town and I've disinvited her but now she's telling people we're alienating her."
He works as a teacher and added that he is worried about what will happen to his career if she leaks the pictures.
He also mentioned he has spoken to a close friend who is a lawyer about the situation.
Social media users were furious at the mother, urging him to out her to the family.
"You have every single right to be pissed and to feel violated. I'd never speak to her again personally," one person said.
"She's a horrible person for violating your privacy," another wrote.
Others slammed her as "pervy" and "gross".