If anyone had asked me 10 years or more ago what a cuckold is, I would have said that a cuckold is a man whose wife or live in partner has a baby by another man and he is the legal but not the biological father. As such he may not even know that he is not the biological dad. He may have known...
How does she feel about being fertilized by another man? Have you considered cuming in her just before or after the other man so you don't know who the dad is until later on?