Just to conclude what ive read up above, if, at any point, Cynthia decides to cuck the MC without his knowledge or consent, its only going to be as part of a bad ending. This outcome will (most likely) not appear during regular gameplay. Theres legal issues involved, if they decide to split up over it (both being co-owners, and all). Bad ending, if anything.
Though i do have doubts if im right, given the last twins event for Rai, where Cookie asks the MC where Cynthia was (as the MC didnt reply to Rais messages during the night), and the MC responds with "...", so, who knows.