I got this idea off of MassKinkMeme. I don't know how old this is I just love this idea.

Garrus makes a date with an old friend on the Citadel Post ME3. Garrus and Shepard have been married for however long and he's just not feeling the love like he used. Shepard walks in on him during the act and feels hurt, betrayed and of course she has no idea as to why he would do that or how he could do that to her in general.

Bonus Points for:
The old friend is Tali.
Is from Garrus' POV.

Double Bonus Points:
Shepard has red hair, bright green eyes and freckles. (I love this Shepard!)
Shepard slaps Garrus.

I'm sorry if I don't quite hit this on the head, I'm kinda new to mass effect :3 Enjoy.

Garrus sighed as he looked in the mirror, he had aged well. His markings still stood out well and the scar on the left side of his face had healed well but he just looked tired. He was dressed nice for a change. Swapping his Armour for a more classy attire of blue and red. He just hopes she likes it.
Well Jane did.
He flinches at the voice in his head. He knows what he's doing is wrong but he just wants to feel some attention for a change, and if this is the way he was only gonna do it then so be it. He and Jane had been married for 3 years after the final battle against the reapers. The first two years were pure bliss, he had the woman of his dreams and things were perfect. Then she got word of James Vega's death. Mission gone wrong and she changed. She became cold and distant. There wasn't the same fire in her bright green eyes as there was before, she took more missions off world leaving him to deal with the council and every-time he tried to touch her in the way he wanted she just pushed him away. He knew she was grieving, but she never reacted like this. Not to Mordins death, not to Thanes. It didn't add up. He walked out of the bathroom and spotted their wedding picture on her bedside table. He picked it up, she looked beautiful and so happy. He wanted That Jane back. Not the cold woman that seemed to take her place.
"I knew you couldn't resist the scars." He had joked to her before the picture was taken, she was right in front of him happy and loving. He growled and threw the picture across the room. How the hell did everything go so wrong. He then looked up at the clock ad started rushing at the time. If he didn't hurry now he would miss his reservation.

Jane smiled at her surrogate Krogan son and her best friend that may as well be a brother, also Krogan.
"Thanks for coming back with me, I know you have other priorities I just really need you both right now." She sighed walking along the presidium.
"Any time you need us Shepard." Wrex said.
"It's Vakarian now." She laughed weakly.
"Bah! He isn't worthy of you." Grunt said walking beside the woman he thought of as a mother.
"Grunt." She scolded.
"Never mind that. What's the matter?" Wrex asked his old friend.
"Let's go and get some food. I'd feel better explaining this over dinner." She said sadly.
"If that's what you want. MORDIN!" Wrex shouted his son who was getting a bit to close to the lake. The toddler heard his fathers cry and came bounding to his Auntie. She laughed and scooped him up into her arms, bounding him onto her hip and making him giggle.
"You coddle him so." Grunt said.
"He will still be a fierce warrior. I only coddle him because I didn't get to coddle you." Shepard said in a baby voice making Grunt scowl. Wrex laughed and so did Mordin.
"You should have one of your own." Wrex said. "You would be a good mother."
"I want to adopt, I was talking to James about it before he died. He told me I really need to think about it, he didn't approve of Garrus." She said sitting down at her table.
"The man has sense after all." Wrex laughed.
"You still haven't explained what's got you so upset." Grunt said picking up a menu.
"It's Garrus. Our relationships been strained. I've needed time to think about things, our life. I want a child but I don't think he's ready." Jane sighed.
"You need to talk to him." Wrex said settling Mordin down.
"I know it's just-" She broke off seeing a familiar shade of blue.
"What?" Grunt said looking towards her line of sight and then growled making Wrex turn around and growl aswell.
"Garrus." There he sat looking extremely close with Tali. They were laughing about something, her head on his shoulder and their hands entwined. Jane heard her heart break in two.
"How could he do this to me!" She cried. She then got really angry. She stood up as quick as a flash and went over to their table, Grunt on her heals.
"Having a nice time?" She said icily. Garrus looked up shocked and pushed Tali away from him instantly.
"Jane!" He gasped standing up trying to touch her. A very large Krogan hand stopped him and non to gently. It was then he took aware of who she was with. Jane had been seated with Wrex and Grunt, a few tables from where he was sat. She had seen everything.
"It's not what it looks like." He said desperately.
"How could you?!" She whispered on the edge of tears. Grunt took one look at his strong battle-master and lost it, throwing Garrus into the wall.
"Grunt! Control yourself!" Wrex came over with Mordin toddling behind him. Grunt held back the punch he was going to deliver to the Pyjacks face and went to stand with Wrex.
"Thanks Wrex." Garrus said standing up.
"I didn't do it for you! I did it for Shepard." He said pointing to where she stood, glaring at the turian.
"I wasn't going to cause a scene here but I don't want you coming back to the house. Why?" She said picking up Mordin again.
"You have been so distant. I thought..." He tried to explain but it sounded pathetic coming from him.
"What? That I didn't love you?" She gasped then laughed tiredly.
"No I-" Garrus began but she just walked up to him and slapped him around the face.
"We're done. No excuses. No more us. I want your stuff out of my apartment tomorrow and I want a divorce." She stated.
"It was only one time!" He pleaded. "Please don't do this."
"I don't care if it was the fifth time Garrus. How can I trust you after this? You should know that a partner always has your back, how can you expect me to trust you after this." She said gesturing to Tali.
"Jane." He whispered brokenly.
"Just don't. Goodbye Garrus." She said walking away with Mordin on her hip and Grunt at her heals. Wrex looked at his former friend. He scoffed and then punched him in the face.
"I couldn't let Grunt have all the fun." Was all he said before following Jane out of the building.

It wasn't until he got back to Palaven did it actually hit him the severity of this situation and he cried.