i just wnated to nitpick that Engels was never married to Mary Burns. However, he was married to Lizzie Burns, Mary's younger sister. He only married Lizzie on her deathbed, about an hour before she died.

Engels was a bit of a womanizer, like he slept with Sibylle Hess, who was the wife of a guy he hated Moses Hess. I'm personally of the opinion he's the one who got Helene Demuth pregnant, the Marx family's housekeeper.