One of Huxley's major critiques of the brave new world is that its society has left no room for human relationships. It has tried to squeeze all the emotion out of human life. You can see this, for example, in its attempts to take the grief out of death.
You can also see Huxley criticizing this by his use of promiscuity and sex. In the brave new world, sex is only about fun. There is no romance or love involved. By promoting sex without emotion, the society is trying once again to get rid of the most human emotions we have (in this case romantic love).
So, the promiscuity in the book is part of Huxley's critique of the society's attempt to eradicate human feelings.