If you were going to think situationally I would think you could do a lot better than the standard faire of incest and adultery to get a good forbidden feel. Why not think historically? Racially forbidden like mixed racial couples in the 40's or 50's. Religious forbidden like a Roman Catholic knight during the crusades falling in love with an Islamic woman. Politically forbidden like a Princess sleeping with commoners (back when royalty meant something). Forbidden can be achieved through such better means than over used incest and adultery.
All I was trying to do was go beyond some of the same old, same old scenarios that pop up, but you're right: one method would be going the route of historical reference and the tension of that ... .
I still wonder about some situation or some setting that doesn't get used much. ...
perhaps a twist would be the happy family travelling to Africa. The husband already anticipating the possible seduction of his wife by an African. 6 months later his wife is addicted to her lover, who then starts on the rest of the family, including him if he wants his happy family back..