In Ultimate X-Men, Cyclops once quit the team and joined Magneto's Brotherhood for a number of reasons including Wolverine and Jean Grey hooking up.
It's not easy being Cyclops. As the leader of the X-Men, you have the weight of completing your mission and the lives of everyone on your team to consider. You emit powerful optic blasts from your eyes, requiring you to wear special ruby quartz glasses 24/7. Not to mention, you're worried about fan-favorite Wolverine making moves on your love Jean Grey.
Many versions of Cyclops have had to deal with this, but one version of Scott Summers said enough was enough and quit. But he didn't just quit the X-Men, he went and joined with Magneto and his Brother of Mutant Supremacy (known as the Brotherhood of Evil Mutants in other universes).
The first arc of Ultimate X-Men - written by Mark Millar and illustrated by Adam and Andy Kubert - took place in an alternate Marvel universe where Professor Charles Xavier recruits a group of teenage mutants called the X-Men to fight against his former friend, Magneto and the Brotherhood. In an attempt to establish good will with the US government. which had employed a deadly Sentinel Initiative to locate and stop mutants hiding in plain sight, the X-Men attempted to rescue the President's daughters from her abductors. They were successful but their victory came at the cost of X-Men's Beast, who almost died during the mission. Although Professor X and Jean Grey are able to save his life, his near-death experience weighs heavily on team leader Cyclops.
As if that wasn't bad enough, Wolverine is seen talking with Jean Grey, the girl that Scott is in love with but has been too afraid to properly ask out. After some back and forth flirting between the two, Logan asks Jean how she feels about him and despite her mixed feelings, she admits to having an attraction to him and they finally kiss with Cyclops witnessing it from one of the school's windows. Not only is Logan enjoying the moment but he can't help gloating that Scott is getting a private show.
While that may have been the final nail in the coffin, the hammer really comes down when Scott announces his resignation to Professor Xavier. While an angry and exasperated Scott has become disillusioned with Xavier's mission to work with people that have been putting mutants in danger, Xavier comes off as cold and calculating as he attempts calm Scott's temper with some gentle psychic manipulation. Countering that with curse words that were so foul Marvel couldn't even publish them, Scott storms off and flies the Blackbird to the Savage Land, a seemingly prehistoric land where Magneto and his followers live in a private citadel. There, Magneto goes to greet Cyclops and welcome him into their city as the newest member of the Brotherhood.
Although Scott's time with the Brotherhood would be short, one can say with almost certain that the real reason he left was his frustration over Wolverine and Jean Grey hooking up. Beast almost dying during their first official mission was one thing, but seeing Jean and Logan making out was the final straw.
Although Jean ended her relationship with Wolverine, tensions between the three mutants would continue throughout the Ultimate X-Men series as Cyclops and Wolverine's feelings for Jean Grey resulted in a near fatal battle (which almost cost Wolverine his own membership on the team).Love sometimes makes people do stupid things but rarely does it ever get you so mad that you end up working for the bad guys just to make a point.
