“What will Trump do when they start bringing up his infidelities?”
Start bringing Juanita into campaign appearances (i.e., hit back 10 times harder).
Trump is not the poster child for marital fidelity himself so I think Clinton would welcome this tactic. Trump can't bring up anything that hasn't been brought up before. It makes Hillary look like the injured party and that may increase Trump's already high negatives among women. It opens Trump up to counter stories about his own infidelities, which he isn't going to be able to leave alone. And time spent on this is time not spent attacking Hillary's record and positions. You can say that Trump can bring up both but which do you think the media will fixate on?
This is a no-win or, at best, a marginal win tactic for Trump. But it does appeal to his personality so he'll likely stick with it. And since it will only serve to make Ted Cruz seem more above the dirt then I think Trump should continue.