Peta spokesperson Lisa Lange said the scene was more serious than it seemed.
"It may seem fun, but the mares are tethered during the process with no means of escape and the stallions are put on a lead rope and dragged to the mares to be mounted. Neither the stallions or the mares have any choice, it's assembly line breeding for profit," she said.
"There's an overpopulation crisis in racing as most horses will not win races, and are therefore discarded and sent on hideous journeys by truck in all weather to Mexico or Canada to be turned into meat."
Mike Judge, the creator of the show which is now in its third season, confirmed the scene wasn't faked, but claimed HBO asked him to include more of it in the episode.
"I actually have an email (saying), 'I can't believe I'm saying this, but can we have more screen time with the horses having sex?'"
Many fans took to Twitter to comment on the scene, but most saw the funny side.
* Silicon Valley screens on SoHo tonight.
