Promise?

DNA test

Dean stared at her, "What do you mean?" He mumbled.

Tears welled up in Mary's eyes. "Dean, I am so, sorry."

Dean narrowed his brows in concern, "Hey, hey, look, if it's something you're not comfortable sharing then it's totally fine. I'm not gonna force you into anything."

Mary shook her head, "No. No, you deserve to know this. I - " She gasped, "I owe you, this truth."

Dean nodded slowly before motioning for her to sit. She closed her eyes and took in a deep breath as she sat beside Dean. "John was ecstatic when he found out I was pregnant. He was looking forward to forming a family, being with you, take care of you...love, you." She mumbled the last few words. "But I was worried, because...I didn't know whose child it was, his, or...Bobby's." She took in a shivering breath when she saw tears well up in Dean's eyes. "So I somehow took a paternity test without them knowing, to find out who the father was." She paused for a moment, "It was Bobby's." A single tear fell out of Dean's eyes. He kept them closed while he heard Mary talk. 

"I thought for months on whether or not to tell John and Bobby about it. But then, one day, when I wasn't at home, John went through my stuff and saw the paternity papers. As soon as I arrived home, I saw both, Bobby and him, fighting with each other. Immediately, I knew what was happening, and I tried to break the fight. Bobby wanted to be your father, so badly," She whimpered, "But John got furious at the mere suggestion of Bobby even partly being your dad. Somehow, we got to an agreement that if you ever found out that Bobby was your dad, John would let you go, and till then Bobby would stay away from you. Bobby obviously wasn't okay with that conclusion, but John threatened him that if he ever broke the deal, he would...kill, me."

They were silent for a few minutes before Dean asked, "Is that why dad hates me so much? Because I'm not genetically his kid?"

Mary winced at the pain in his voice. She whispered, "Yeah..."  

Dean ran his fingers through his hair. He groaned, "This is a lot to take, mom." He let a few tears slip through his eyes. His breath shuddered as he buried his face in his hands.

"It's okay if you hate me, Dean. I know that it was a mistake keeping it from you. And...I'm sorry, for that." 

Dean sighed, "I don't - I don't hate you...okay? I understand why you never told me about it."

Mary nodded understandingly, "Do you want me to leave?"

Dean groaned, "I'm not gonna kick you out, mom. I told you this before and I'm telling you this again. I'm not dad. I'm not like dad. Hell, I'm not even his real son." He chuckled at the end of his sentence, "You know...I hate to admit this out loud, but I'm glad I'm not his son."

Mary smiled ever so slightly, "I'm glad you're not his son too."

Silence fell upon them, but this was a comfortable one. Mary felt like a weight had been lifted off her shoulders. She had kept the secret to herself for so long, she hadn't realized how much it was bothering her, and now that she had told the truth, it wasn't a secret anymore. 

A thought came to her mind, "Do you...wanna meet, him?"

Dean looked at her. A soft expression took over his face. "Will that be okay - will it - will it be okay? Would he be okay with that?"

"Of course. Why do you think his number is on that book?"

"Why?"

Mary smiled, "I went to Bobby's that day. The day I told John I wanted to divorce him and ran away, I had nowhere to go. So I called Bobby. I couldn't call Sam cause I knew he would be in class during that time of the day, and I didn't have your number cause you had deleted it from my phone - "