What if
Jacob did not cheat on Cmdr. Shepard?
Hear me out - used
a youtube video for the information below.
Shepard stumbles into Jacob, wants him to acknowledge her and he shifts uncomfortably as a result of this.
He tries his best to ward her off with an excuse. Then tries to stop her from talking any further. Then loses his composure and says he loves her only to correct it into
past tense while not being able to look her in the eye. He's ashamed.
One more attempt by Shepard and Jacob blurts out
not that he won't but that
he can't.
He then mentions he met Brynn in the Mumbai cell - a year ago.
Remember this timeframe.
Jacob also tries to explain how he messed up but can't find the words for it.
In talking to Brynn herself Shepard mentions she was in a relationship with Jacob - Brynn
knows this and she tells Shepard she's sorry about that.
Brynn then tells Shepard that she met Jacob in Mumbai and she went after Jacob there and then.
She also mentions that Jacob is still in love with Shepard. She then bluntly informs Shepard that Jacob is hers and Shepard should just understand that.
Now for my reasoning why Jacob didn't cheat on Shepard.
This has to do with the timeframe in which Jacob and Brynn met.
You get to talk with Tali on the Citadel way after dealing with Jacob and Brynn. Tali mentions it was three years to the day since she got to the Citadel and met up with Shepard in Mass Effect 1.
Two of those years Shepard spent being dead being reconstructed during Project Lazarus.
And then half a year she spent under house arrest due to collaborating with Cerberus and/or commiting a war crime by blowing up a Mass Relay and snuffing out life on an entire planet.
That leaves six months for the events you get to play in ME1, ME2 and ME3 combined.
Two months for ME3, six months for house arrest and two months for all of ME2 - a total of 10 months ago that Shepard first met Jacob.
Yet Jacob first met and hooked up with Brynn 12 months ago.
To put in other words; Brynn hooked up with Jacob when Shepard was still a corpse/vegetable in Project Lazarus.
Space Seductress Shepard only got her claws on him afterwards; during ME2.
Jacob came across as tragic character in my opinion.
I'm guessing during Shepard's incarceration he came clean to Brynn on his affair with Shepard. This is how Brynn knows about Jacob and Shepard.
Jacob most certainly does not want to follow in his father's footsteps by walking out on the mother of his child (he tells Shepard this back on the Citadel) - even though it means he has to forsake any contact with the woman he fell in love with.
All this seems in line with his honorable character as established in ME2.
In fact I heard Jacob breaks off any chance at a ME2 romance with him if Shepard pushes him too hard after his loyalty mission. That could be because Jacob, in light of his father's life and fate, remembers where he stands with Brynn.