John Kennish - Switched at Birth
Good news, nobody cheated! The hospital switched the Vasquez and Kennish families' baby girls. Angelo found out Daphne wasn't his biological daughter fairly early, but John didn't know until a teenage Bay requested DNA testing.
Robert Baratheon - Game of Thrones
Given the number of bastards he fathered, King Robert wasn't shy about spilling his seed. Siring an heir should have been easy for him. However, he was married to Cersei, and they were not a match made in heaven. In retaliation for his infidelity and other sins, Cersei chose to bore her twin brother's children instead.
Finn Hudson - Glee
Quinn was dating quarterback Finn but seeing Puck on the side when she got pregnant. Quinn knew Puck was the father, but since Finn was the more dependable one between the two, Quinn told Finn the child was his. The truth eventually came out -- in time to jeopardize to New Direction's chances at sectionals.
Rome Howard - A Million Little Things
Rome and Gina never wanted kids, which is why he freaked when he saw the used pregnancy test in their apartment. He was starting to warm up to the idea of being a daddy when Gina told him the test wasn't hers. It was Delilah's.
Warren McGinnis - Justice League Unlimited
Cadmus hijacked Warren's flu shot appointment to inject him with a nanite solution, which overrode his reproductive material with Bruce Wayne's for the purpose of having Warren father the next Batman.
Commanders of Gilead - The Handmaid's Tale
Most of Gilead's commanders are sterile. However, it's forbidden to say men aren't fertile, and the blame falls upon the handmaids. If a handmaid wants to avoid getting sent to the colonies or whatever other brutal punishment Gilead could devise, they have to find someone else to impregnate them.