Yes there is. There's the dialogue between Shepard and the two of them. He never mentions Brynn in ME2, and in ME3 he never mentions her as anything other than a colleague before the events in ME3 (he says they grew close after she contacted him while Shep was in prison) and once more: Brynn straight up tells you that she stole your man. Why would she put it like that if she came first?
In fact, he says he's known Brynn for a year. Shep was in custody for six months. You unlock Jacob's mission after Tuchanka so it's been a few weeks after Shep got released. There's no way he met Brynn first and had a relationship with her. It doesn't fit the timeline unless he met her during the events of ME2 and were playing both women at the same time. Shepard's a badass, but I think even Shep would have a hard time completing the events of ME2 (including DLC) in less than six months. And y'know, the reapers strike in autumn (based on Turian reports, they declare war on 9/28). That gives Shepard a maximum of 2 -1,5 months to complete the events of ME2 (because that game begins in 2185).
Bottom line, your version makes no sense.
Miranda doesn't mention the men she sleeps with either, that doesn't mean she hasn't slept with people which we know is true due to info the Shadow Broker has. When someone is having multiple relationships, they tend not to tell the others about them or lie about it if caught. But to clarify, I don't think they were lovers before Shepard, I think they were dating before Shepard. Hence why Brynn says what she does because Shepard was the first to get the priiize even though she was the second woman involved.
Why not? The events of ME3, which is a lot larger scale than what ME2 was, was done in six months. You're telling me it is less believable for Shepard to beat the Collectors, a pawn of the Reapers, in six months than it is for the Reapers to be defeated in the same amount of time?
In short, Jacob is a lying jerk if you romance him. Speaking of lying jerks, let's get back on topic and talk about Varric. Or is there anything else that you want to tell me I'm wrong about, even though you criticized me earlier for supposedly doing the same thing?