Why did Philip never suspect that Joy's second child was his?

Just before Philip and Joy made love in Genoa, Philip asked Joy is she was protected, and she said that she was. What she tells him upon their second meeting years later, however, is that she lied; she had gone off the pill as she and her husband were hoping to conceive a second child, and she was afraid if she admitted the truth that Philip would stop what he was doing; Philip never suspected for a moment that he could have impregnated her.