This is a list of all characters who appeared or were mentioned to have had a romantic relationship with Jack McFarland. Jack has been known for his various flings throughout the show with most of them being mentioned only, and with a few exceptions, Jack never seems to take them seriously or to settle down with anyone for very long.
Before the show
Rudy is the Jewish cowboy Jack met on the internet. Will implies he advised Jack against dating him.
Alex is a graduate student working on his dissertation when he met Jack at the bookstore. According to Jack, their hands touched as they both reached for a copy of "The Making of Beaches". Jack, however introduced himself to Alex as a lawyer and used Will's office to pretend to be the "stable, mature, goal-orientated, responsible" kind. At Jack's urging, Will pretends to be his assistant. Alex however, feels insecure of Jack's supposed success and breaks up with him right before asking to be set up with Will who he thinks is as "lost and aimless" as he is.
Bill is the head of a gay conversion therapy group called "Welcome Back Home". While passing flyers at a gay club, he meets Jack, who is immediately smitten. At the next meeting, Jack and Karen pretend to be converts so he could hit on Bill.
Björn is a Swedish guy who guy dated. Jack recalls that Björn claimed he could bench-press Jack's weight which subsequently led to "a very painful breakup". It was also Björn who originated the idea for The Subway Tush, which Jack stole after Björn said he was moving back to Sweden.
Fernando is a guy Jack dated while he was married to Rosario. Though he is somewhat slow, Fernando can lick his own eyebrows. Jack dated Fernando the same time he was also going out with another guy named Dennis.
Walter dated Jack before Will "fooled around" with him. After Jack found out about him and Will, he admits that Walter is the "love of his life" and how devastated he was when they broke up. To make it up to Jack, Will sets him up with Walter again but Jack admits he has mistaken Walter for somebody else and leaves immediately.
Mipanko is a guy who stayed with Jack at Will's apartment after getting to know him for 11 minutes. Mipanko's father also slept on the couch. Jack told him he's a Navy SEAL.
Paul is a guy who works at the coffee shop Jumpin' Java and unknowingly gets Jack addicted to caffeine.
Rocco is a guy who went out with Jack for two days and then broke up with him. Jack's depression over the break-up inspired him to start writing his novel "To Weep and to Willow: A Harlequeen Romance", which Karen loved. After they got back together, Karen bribed Rocco to dump Jack again so he can continue his novel.
Jack's longest relationship is with Stuart, which lasts several months during the sixth season, until Jack cheats on Stuart.
Travis is guy from the cowboy bar Jack dated whom he says is "hottest guy [he's] ever gone out with". After Jack introduces him to Will, Travis hits on him, saying Jack is just an idiot and he is out of his league, which angered Will and caused a bar fight.
Lenny is an agent of the US Secret Service who works at the White House. It is implied that he had a relationship with Jack from before, so he helps him and Will have access to the White House to meet a congressman. Lenny holds the distinction of being Jack's first onscreen kiss.