Hey! So. LisaTime! Sorry about the shortness... The cliffy was too good to resist.
The question's obvious.. How does she know?

I was a little scared by the sudden quiet, shaky anger that Lisa was radiating.
'You... Came... Here... To tell me that you're engaged...' Her hands clenched on her lap, and she glared at me. 'Which hurt enough, I'll have you know. Then I find out... That you don't love me... For a man?'
I shook my head, standing. I hadn't come to argue or cower from her insults, but I would not sit and listen to her if she was this worked up. 'Listen, Lisa...'
'No, you listen! Gabe, short for Gabriel, I'm guessing- what kind of man is he? Is he tall, short? Fat, thin? Does he complete you, or does he drive you crazy? Is he loud or quiet?' She glared at me through tearful eyes, and I winced as she spoke again. 'What makes him better than me?'
I shook my head, polishing off my beer. 'I'm not gonna argue, Lisa. I just wanted you there, but if you can't handle it, it's fine.'
Her glare became deathly. 'Of course I can handle it...' She muttered, scowling. 'Though I'm not happy about it. I don't get what makes him so special.'
I smiled slightly, and glanced away shyly. 'You will when you meet him... He's the most perfect guy in the world.'
Lisa chuckled a little. 'Wow, you really do love him, huh?'
'More than anything...' I answered, nodding.
She smiled. 'In which case... I'm happy for you. Congratulations, Dean.' I glanced up, and although pain and humiliation were still clear in her eyes, she was putting on a joyed facade, just for my benefit.
I smiled weakly, and pulled her into my arms. 'I'm sorry... I wanted us to work, I really did. But... I've never felt this way before, about anyone. He...' I shook my head, unable to explain just what feelings Gabriel evoked in me.
She nodded, glancing up at me. 'I still want to meet him.'
'You still can... Sam would like to see you, I can bring them all over tomorrow, if you like?'
'Them all?'
'Yeah, um... Sam has a boyfriend...'
'Sam's gay too?' She chuckled dryly, shaking her head. 'Winchesters...'
'Um, maybe I should tell you now... Sam's boyfriend, Castiel, he's... A little bit older... Than... me?'
Lisa's jaw dropped, and she scowled. 'And you allow it?'
'You haven't met the guy, he's... Good for Sam.' I winced, even contemplating Cas and Sam's relationship.
Lisa nodded. 'So long as he's happy... He had a rough time last year, didn't he?'
I nodded, sighing, then froze. 'How d'you know about Sam's year?'