Random interpersonal stuff as it occurs to me;
In the reboot, Cosmic Boy and Saturn Girl dated, instead of Saturn Girl and Lightning Lad.
In the classic continuity, Saturn Girl and Timber Wolf had a moment (more a hug, not an affair or anything), and it led to the breakup of
Brin and Ayla (but doesn't seem to have significantly affected Garth and Imra's marriage).
Shadow Lass used to be interested in Brainiac 5, before settling on Mon-El. Later they broke up and she dated Earth-Man for a brief time, before he died and she went back to Mon-El.
Triplicate Girl was very into Superboy, before choosing Bouncing Boy instead.
In the Reboot, Phantom Girl and Ultra Boy's relationship fizzled due to jerkishness on his part and she ended up with Timber Wolf.
Colossal Boy had a crush on Violet, ended up with Violet, only discover that the Violet he dated (and secretly married!) was actually a Durlan infiltrator, and things got a wee bit tense for a long time between them and the real Violet, when she returned. The Durlan infiltrator, Yera, has since joined the Legion as Chameleon Girl, but her and Violet are... not friends.
Sun Boy dated Laurel Kent for a hot second, and that Laurel Kent turned out to be a robot Manhunter out to score an invite to Legion HQ so it could blow up the team. He's also apparently pissed off Dreamy and Tinya and Dragonwing, in the most recent Levitz reboot, because they, and all of the women on the team, seem to push him away or express disgust around him.
In the Threeboot, Ayla had dated Sun Boy and Ultra Boy, in the past. Shadow Lass had dated Karate Kid, and was currently dating Ultra Boy, and it was flat-out stated by Karate Kid that she had just been using him to level up her hand-to-hand skills, and dumped him when she felt like she'd learned enough.
There are tons of others over 60 plus years and four or five continuities, but that's the first batch that comes to mind.