I figured there would be more to the story than what was posted, so I decided to check where you are, and your profile says India. If that's the case, then I can understand more why this has come as such a shock. Despite being raised in the United States, I am still Mexican and was raised with those values, so I can understand more or less what you're feeling. You need to understand though that this is quite natural. It doesn't at all mean that he loves your mother any less or that he's even any less attracted to her. It doesn't mean that he will be unfaithful to her either. As long as it doesn't take over his life, you thankfully have no problem. He's still the father you love and know, now you just know he still has urges like any other man.
You know, this could actually be a good thing. You might want to tell him privately when no one else is around what happened and tell him to be a bit more careful next time. Be loving and understanding, don't judge him. If he knows he has that from you, it could actually make it much easier to tell him that you're trans when you're ready.