Fifty years ago, science fiction stories were allowed to include sex if it was
necessary to the plot. Because that requirement was laid on some of the most imaginative people in the world, there were a lot of ways in which sex became necessary to the plot.
Times have changed, and now sex in sf is somewhere between gratuitous and mandatory, just as in the mainstream. Now only
homosex has to be
necessary to the plot.
Thanx to
Making Light.