I normally try to type gender-neutral, but for this one, I think it works more likely with the genders selected.
A man that is not inclined to incest does have at least 1 attractive female relative that is attracted to both him and his girlfriend. The girlfriend in question is bi, and the female relative seduces her first, and then gets her to suggest a 3some without telling her bf who the second woman will be.
For this next bit, there are many ways this could happen, this is just one:
The two begin a sexual evening as a couple, and the girlfriend gets him to lie on his back, and mounts his face. As he is giving the girlfriend oral, the female relative enters and mounts his cock. He doesn't even find out until he has cum at least once who was mounting his cock.
It could be entirely innocent, too. The gf could have met a relative that was divorce, but still using her ex husband's name. This could be taking place at the gf's apartment, such that neither relative knows the other before one or more orgasms have happened. Maybe the gf has had a relationship with both the related folks for a while, and been dying to introduce them for 3somes.
A man that is not inclined to incest does have at least 1 attractive female relative that is attracted to both him and his girlfriend. The girlfriend in question is bi, and the female relative seduces her first, and then gets her to suggest a 3some without telling her bf who the second woman will be.
For this next bit, there are many ways this could happen, this is just one:
The two begin a sexual evening as a couple, and the girlfriend gets him to lie on his back, and mounts his face. As he is giving the girlfriend oral, the female relative enters and mounts his cock. He doesn't even find out until he has cum at least once who was mounting his cock.
It could be entirely innocent, too. The gf could have met a relative that was divorce, but still using her ex husband's name. This could be taking place at the gf's apartment, such that neither relative knows the other before one or more orgasms have happened. Maybe the gf has had a relationship with both the related folks for a while, and been dying to introduce them for 3somes.