Man, do I love plot bunnies. This one jumped on me and wouldn't go away, nibbling at my brain...
So this takes place sometime after City of Glass, but only because of a mention of Sebastian/Jonathan. Max is alive (As he is in all my TMI stories, because I can't bear him dead), and The Kiss (You know the one) never happened.
I don't own TMI. I think you know that.

Alec shrugged off his sweater as he prepared for the day's training. He entered the training room and tossed it on a hook, then began stretching next to Jace; once upon a time being so near a shirtless Jace (they usually wore just sweatpants for training, for practical reason) would have terrified beyond belief. But now Jace was back to being just a brother; he had the image of someone else shirtless to occupy his time.
Just the thought of Magnus made him blush, remembering last night, when Alec had snuck out to see him and hadn't returned until much later. He had barely gotten any sleep; training would be a nightmare.
And it was even worse because Maryse Lightwood was right there, with a pad of paper and a pencil. "Hello, Alec. I'm just going to be making sure your training is going well, alright?" She said without looking up.
Alec nodded then, realising that she wasn't looking at him, said,
"Alec, are you okay?" Jace asked, looking a bit concerned at Alec's slight limp. He looked him up and down, and smirked. "Oh."
Izzy strode in wearing a t-shirt and shorts. "Have a good time last night?" She had caught Alec sneaking back in the morning. Not to mention the fact that he only spent around one night a week away from Magnus.
"Uh, yeah. I slept well." He stammered. As far as he knew, neither of his parents knew about the outings.
Jace smirked. "So you'll have lots of energy. Good."
Alec groaned. He so wasn't in the mood for this. The one night he doesn't top... He was sore all over. And of course Jace would insist on focusing on kicks. If he was a mundane, he wouldn't even be able to walk right now.
Maryse finally looked up. "So, what will you be starting-" She stopped short when she took a look at Alec. "Alec! What happened?"
Jace snorted. Izzy covered her mouth and winked at him, and Alec glanced down. What he could see of his chest was covered in shiny purple... oh. Crap.
"I... uh..."
Maryse's eyes widened. "When was the last time you went hunting?"
"A week ago." Jace supplied helpfully.
"They should have healed by now!"
Izzy giggled. "I thought I heard the door open last night."
"I hate you." Alec grumbled.
"A door - did you sneak out?!" Maryse shrieked.
Izzy and Jace exchanged a glance worthy of Jonathan Morgenstern himself. Alec didn't know why they were suddenly ganging up on him... Either it was too hard to resist with his current situation, or they had finally decided to toss him out of the closet. Headfirst. Onto a few upended swords. It made him furious - what gave them the right?! Were they getting impatient, like Magnus had a while ago? But he had gotten over it... right?
"It's been happening for a while. Once I went into Alec's room because I heard a noise downstairs, and he wasn't there." Izzy supplied.
"You have been sneaking out, haven't you! Are you getting into mundane fights?" Maryse shouted.
Alec mumbled something under his breath.
"Speak clearly!"
He repeated himself, a bit louder. She scowled.
"Oh, for the Angel." Jace groaned. "He's never going to say it. Those aren't bruises."
"They aren't..." Maryse thought for a split second. "Then... ALEXANDER GIDEON LIGHTWOOD!"
Perfect. Alec thought. "Mom, I can explain."
"Explain what? We've had enough trouble with your sister and her - her trysts with Downworlders and mundanes; you were always the responsible one! Now your father and I have to deal with you sneaking out to who-knows-where doing... I don't even want to think about it! Were you at a mundane bar? Or a club? How long has this been going on?" She ranted. Alec grimaced. If he stopped her now, she would just get even angrier. And if he told her the truth, she would ask even more questions. But that would postpone the anger, so he went for the lesser of two evils.
"I'll tell you everything, just give me a second."
She seemed to explode. "Everything? What is there to explain? You've been sneaking out, directly disobeying our orders! Alexander, you are grounded. Grounded until Max gets married. You are only to leave the Institute on official business for the Clave and on hunts, accompanied by another Shadowhunter."
"MOM!" Alec shouted. "Would you let me talk?" Maryse finally took a rest. "Yes, I've been sneaking out. For months now, actually. But it's not some string of random Downworlders, or mundanes. It's a very respected Downworlder. You've met him before."
She sighed. Some of the initial anger and shock was wearing down, but she was still suspicious. However, one of the key words didn't register. "A respected Downworlder? Who is it, a werewolf pack leader? I don't know any local ones. Or a vampire queen? Please tell me it's not a vampire queen."
"It's not a vampire queen. It's one of Lilith's Children." Now that Alec knew his mother had ignored his confession once, he couldn't bring himself to do it again.
"A warlock." Maryse frowned. "I don't know very many warlocks, let alone nearby ones. There's Catelyn Branwell, up in Salem; Lynn Rans is the High Warlock of Newark... Or Tessa Gray, she comes to New York sometimes."
He almost laughed. "You're not going to guess it, Mom." She stopped. "It's the High Warlock of Brooklyn."
"Magnus Bane? But... Oh."
"Yeah. I'm gay." Alec smiled, glad that it was finally out. Izzy and Jace smirked.
"You've been sleeping with Magnus Bane?"
Alec blinked in surprise. "That's what you got out of that?"
"Answer my question." She said.
"Uh, yeah." He sighed. "Yeah, I have." At that precise moment, through some miracle, Alec's phone buzzed, in his pocket on a hook. It was Magnus. "I should get that..."
Maryse huffed. "Fine. But I want to meet the warlock who's been making my son lie to me."
"Mom, it was mostly my idea."
Right, my idea. I was the dominating one in this relationship. Usually I was the one to bite at his chest, sometimes softly, sometimes not. I was the one to slowly drag my tongue over the skin on his neck, the one to tease him endlessly until he was ready to burst, then I was the one to make him burst.
"Hello?" He said into the phone, picking it up.
"Alexander! Darling! Are you done training with those awful people? I've been itching for a repeat of last night's luck." Said Magnus. He could picture him grinning, blindingly bright.
"Magnus, this really isn't the time."
He gasped, over-theatrically. "Not the time? Why, it's always the time!" He said nothing. Maryse made a little 'get-on-with-it' motion. "Are you okay?"
"Your 'luck' just caused me some major trouble." Jace made a gagging noise at the word 'luck.' "I had to tell my mom about us."
Magnus was silent for a few seconds. "Alec. How did she take it?"
Alec smiled a bit more. "She wants to meet you."
"Well, shit." He scowled.

And that's it! Hope you enjoyed it! Please review if you did, and if you didn't, and if you went WTF.