 He was trying to make a son-in-law, so this is where we go back to those stories of the Sabine women. The closest thing there is to a son is a son-in-law, and he wanted Marcellus to be his heir. He's the daughter of his sister Octavia, he's the son of his sister Octavia, and he wanted to make him his heir by making him his son-in-law, the closest thing he could to his son.