I am guessing his agent said hell no to resigning. If he resigned his contract might be voided. I assume he has a morals clause but will hold out for some sort of buyout if the rumors of multiple affairs are actually true. No way he can come back from sleeping with a VPs wife unless he has won multiple rings as a coach.
If they don't fire him he'll be required to take weekly classes in sexual harassment in the workplace and go through sexual addiction rehab which celebrities go through like James Franco and David Duchovny....long list but it will make sure he keeps a paycheck although a much smaller one and probably has to pay a fine as well. I just don't see how the other two employees can coexist with him from here forward so I'd see him resigning once he's milked a reduced paycheck instead of getting fired without compensation. They might buy him out...hard to say but Boston is an aggressive fan base...might not get much fan love in that city from here on either. He needs a change of scenery if he's going to coach again. Boston should fire him. It would probably save them money at this point to just cut bait.
Was his partner underage? Was his partner forced into sex without their consent? If the answer to those questions is "no", then why is it anybody else's business?