I don't think that the intent was for realism and usually isn't in these games. However, there's always exceptions to the rules and sometimes wives do blatantly cheap on their spouse. In a blackmail situation you may not be thinking clearly. After getting more and more pleasure rewards for the sex you end up looking forward to it and the adrenaline/excitement/fear/shame may also dull your decision making. Crazier things have happened in real life, but I think part of the appeal of adult visual novels/games is where you can detach from the consequences and normalcy of reality. It is sort of like a drug hit except that you're body's naturally supplying the drugs.