The thing with this situation is we have no idea what has happened, not our business. We don't know if Watson is husband of the year, or a complete and utter bastard. May could be comforting a desperately unhappy woman. Or maybe it was a drunken one night stand. Who knows and who is in a position to judge? Who is right and who is wrong is never easy and often many shades of grey.
To be fair to the Roosters, if the allegation is true, they had no choice but to get rid of of one of them. It's untenable.
Since Watson is the person in the relationship it makes sense they release May. That said, maybe they thought they can replace May easier than Watson, who knows? It's a mess, but let's not rush in labelling May a villain, when we are not privy to the facts.