I realize that I'm a little late to the "Fuck Tara: Yes / No" debate, but I may as well toss in my two cents.
Women, from personal experience as well as debacles I've seen others suffer through, have a tendency to set guys up. They may not even realize that they're doing it. A situation like Jack's, where Kayla has given him the green light to plow Tara, is, subconsciously, a test of his fidelity. Even if Kayla's intentions are good (debatable), she may experience serious buyer's remorse after the fact. That's one thing women can do that men can't - retroactively change their mind and feel justified in their rising anger. Dudes can't do this. It's not in our DNA.
That said, if you do decide to have Jack and Tara get it on, in order to be realistic, you'll have practically zero but to have Kayla use it against him. That's how this shit works.