In the threeboot, IIRC, but she later was revealed to have been using him to learn some cool martial arts moves, and moved on when she had leveled up her hand to hand skills. Classic Shady was all about Mon-El, after a brief period of interest in Brainy that went nowhere. (And a later fling with Earth-Man, that we'll pretend never happened, because he died and she was back with Mon-El .0037 seconds later, like nothing happened.)
That all said, I have zero clue about Reboot Shady. I was trapped in an alternate dimension without comic books for most of that time.
In my reading I'm interested to find out when the Mon-El thing started and which writer initiated it. Maybe that's one reason I never was a fan of Shadow Lass--because she seemed all wrong for Mon-El in my head.
It's interesting how some of these romances developed. Lone Wolf and Light Lass had a big romance that was the crux of his first appearance. But Brin completely disappears from the comics until Shooter's Adult Legion story--where the two are old marrieds and (to me) they look like Walter Pigeon and Greer Garson--and he's identified as Timber Wolf. Then he's not mentioned again before he joins the Legion (in a story I will be reading soon) along with Chemical King--presumably they were in the Legion Academy. Shadow Lass was inducted in the Legion on the battlefield as it were, by Superboy--yet Timber Wolf had to prepare all that time before he could get membership. But if off-panel and behind the scenes he was at the Academy, I imagine he was courting Ayla that whole time.
It's too bad that Night Girl was so stuck on Cosmic Boy. Even though I liked that romance, it's so cute how John Forte draws Night Girl and Polar Boy. She appears so tall and he so short--and they are the bulwark of the Legion of Substitutes--I guess they were always just good friends, but a pairing between the two would have been fun.
Night Girl is fun because, during that time, it seemed like a good foot of her height was that towering hairdo.
But yeah, that whole relationship dynamic was sketchy, even for the time it was written. "I like this boy I heard about on the news. I'll have my scientist dad experiment on me to give me super-powers so I can fly off to another planet and join his super-hero team and marry him even 'though we've literally never even spoken and he has no idea I exist!" Yikes. Slow down, Lydda! And what does it say about her father that he's all like. "Good plan! Get in this machine I just made out of a toaster and some leftover pizza crusts!"
Actually it didn't take long to find the answer to this, as it happens in Jim Shooter's Mordru story. Since Luornu, Tasmia, Lar and Clark are the four Legionnaires in Smallville--Luornu laments her unrequited affection for Superboy and fumes over how Tasmia has transferred her interest from Brainiac-5 to Mon-El.
But Tasmia realizes that Luornu likes Clark and does what she can to get him to notice her.
And, yes, Duo Damsel says in her thought balloons that she knows who Superman marries. However, I'm not sure why that should stop teen-agers from having fun. Not everyone we date has to be someone we'll marry.
In any case, the Legionnaires know about multiple Earths. In a previous story it shows Chameleon Boy on another Earth, where he's hanging out with his identical counterpart there. Surely, Duo Damsel could just go to another Earth and hit on that world's Clark Kent.
In my head, Mon-El has a thousand-yard-stare going on from his time in the zone, and Tasmia is exactly the sort of dynamic take-charge person he needs, after a thousand years of being a passive observer who could use a little push, from time to time, to keep him from retreating back into himself. (It's also entirely possible that after 1000 years of the only women he could really interact with were Kryptonian criminals sentenced to eternity in the zone, like Faora, he might have picked up a taste for bad girls...)
Yeah, but in the time when this was written, it probably seemed more like 'one true love / marry your high school sweetheart' was more the ideal they were shooting for than 'Lu would be happy to be Clark's twenty-third Tinder match.'And, yes, Duo Damsel says in her thought balloons that she knows who Superman marries. However, I'm not sure why that should stop teen-agers from having fun. Not everyone we date has to be someone we'll marry.
Plus the writers of the day weren't going to write anything that contradicted his 20th century romantic shenanigans with Lana. Most other Legionnaires could get paired off eventually, but not Superboy, he had to remain unentangled and 'available' for his 20th century girlfriend.
Of course, unlike the other writers, Jim Shooter was just a teen himself at the time, so he was writing what he knew. His teens behave more like the real thing.