Romance between brother and sister. It happens more often then you think. And when it does it is often intense.
It is a myth that all incest involves a parent or other adult raping a kid. When in fact most incest is consensual and between teens and/or "adults".
A female singer who 1. after her show she picks someone out of the audience to come back stage and have sex with her and/or 2. who makes appointments with members of her fan club to meet and have sex with them.
First started by Madam x who is bisexual.
In some societies a man can be married to any number of wives who can also be married to any number of husbands who in there turn can be married to any number of wives and so on so that the pattern on marrages looks like a network.
some have proposed this for our society.