Cheating on wife with alien girl

So I watched a movie probably 10 years or so, about a couple and I think they found an alien egg or something like that and it turned out to be a normal looking woman, but then later in the movie he cheated on his wife with that thing, looking like a woman and during the thing they did, when the girl was having her climax, she grew a scorpion tail and I think changed her eye color. Then she killed him with that tail by piercing him.

I can't find it anywhere and all of the internet shows me all kinds of movies about alien abduction, alien monster or things like that. Someone help please...