He was struck by a question and had no answer. Staring at her, he kind of hope she'd elaborate. She stared at the ceiling debating what to say.

"Are we dating? Or just having fun? Or what?" She asked only glancing at him for a brief second. Honestly, she was afraid to hear the answers. Did he have feelings for her like she did him? Or was this a meaningless game. They stayed silent for a long time to the point she was about to accept that as the answer. Just as she was about to roll on her side and give him the cold shoulder for the rest of the night. He leaned down and kissed her hard on the lips.

When he pulled back, he gave her a small smile. "I think I'm falling for you." He said quietly. A smile worked its way onto her lips as she rolled on top of him.

"What a coincidence…" She kissed his chest. "I think I'm falling for you too."

--

After a couple hours of not hearing anything out of Mark or Meredith, Addison walked to their room and knocked, waited for a few moments, then knocked again. Annoyed by how long it was taking them, she knocked once more before the door was yanked open.

"What is it, Addi?" He groaned. She looked at him. He was wearing nothing but a towel and his hair was still damp and messed up like he had been running his hands through it.

"Where have you guys been?" She asked looking into the room. To her dismay, clothes were scattered on the floor. "And what have you been doing?"

He rolled his eyes as she pushed past him and walked into the room. She could hear the shower running in that bathroom and looked at him shocked. "Addi, I'll be out in a minute. Can you at least wait until I get dressed?"

"I know something is up, and I want to know what right this second." She said firmly.

"You're screwing her, aren't you?" She said coldly. When he said nothing, she had her answer. "What the hell is wrong with you, Mark? She's my step-sister, for Christ sake!"

"What the hell is wrong with that? Meredith is amazing."

"I don't want to know how good my sister is in bed, Mark. You betrayed me."

"I betrayed you!?" He yelled. "I didn't do anything wrong. And I sure as hell am not apologizing for my relationship with Meredith."

"She's Meredith Grey. Do you know how damaged she is? You are doing neither of yourself a favor by sleeping together."

"Don't you think we should be the judge of that?"

"What the hell are you trying to prove by sleeping with her?"

"What the hell is your fucking problem!?" Meredith yelled as she emerged from the bathroom wearing her bra and jeans. "Because I have no fucking idea. Are you just that damn ashamed of me? Am I not good enough for Mark?"

"I didn't say that." Addison argued.

"Stop patronizing me. Stop thinking that I can't take care of myself. I've never needed your help, and I sure as hell don't need it now."

"I don't want to see you get hurt. I am so freaking sorry about that."

"Now you care about what the hell goes on in my life? Now that I have boobs and blonde hair. Now that you can be seen in public with me and people won't go 'what the hell is wrong with them?' I'm done with listening to you criticize me because I'm just not as perfect as you or whatever the hell you bitch about. I'm tired of you trying to poke your nose where it doesn't belong. And that includes in my relationship with Mark."

Addison looked at Mark hoping he'd say something to refute what Meredith had just said. But he didn't say anything. He just stared at her. For a second they stared at each other until she walked out of the room. After the door slammed, Meredith let out a deep sigh and pinched the bridge of her nose. Mark instantly engulfed her in his arms and her head fell against his bare chest. It felt good to be in his arms and for a long time, she didn't move out of them.

--

He hadn't spoken to anyone other Meredith for a few hours. Once she fell asleep, he slipped on pair of sweatpants and a t-shirt, wrote a note in case he woke up, and left the room. He found himself in the lobby Starbucks surprised to see Addison there. She was wearing some black sweats and a Dartmouth sweatshirt. Her hair wasn't perfectly in place as it usually was which wasn't a good sign.

"Hey." He sighed standing behind her in line.

She jumped surprised herself to see him there. "Hey." She replied quietly.

"What are you doing here?"

"Couldn't sleep, you?"

"Same." He sighed and stepped up beside her to give the guy at the counter their orders then paid. They were silent until they got their coffee and found a table.

"Why are you… sleeping with her Mark?" She asked carefully.

"Because I like her." He answered simply.

"We've been friends for a long time. Why would you risk that by sleeping with her? You know I wouldn't want you to."

He swallowed hard unsure what to say. "For some people, it's worth risking it all…" She looked shocked but said nothing. "Look, Addi, I don't want to lose you. You're my best friend. But Meredith… she's amazing."

"You really like her?"

"I'm falling in love with her."

Tears welled in her eyes. She didn't want to lose Mark, but she knew she already had. It was always going to be either her or Meredith. And he had unknowingly chosen his side. She forced a smile hoping he wouldn't noticed the tears in her eyes. But by the smile on his lips and the joy in his eyes, something told her he wasn't going to be noticing much of anything about her now that he was thinking of Meredith.

"Maybe you could still fix you guys." Mark said quietly.

Addison gave him a sad smile. "Our relationship had been far past repair for a long time, way before this trip, way before you and her."