It is true that sex and intimacy on the astral planes has been going on since humanity has first manifested into a physical form upon earth.
Many shamans and witches or energy practitioners, have come across increased sexual energies when working on the astral planes.
Sexual energy is a lower octave of spiritual energy. In sexual Tantra practice, the ultimate goal is for both partners to achieve God/Goddess identity and higher consciousness.
This unifies all energies involved, for they are exchanging sexual and spiritual energies that bond their internal opposites of masculine and feminine principles.
Every man has internal female energy aspects and every woman has internal masculine energy aspects, that when fully bonded together cause a third powerful force on the astral planes and to the higher dimensions. In Tantra there are various stages of this masc/fem bonding of energies and this does assist in balancing subtle body energies and character qualities also.
However, having intercourse in the astral plane will affect energies on the physical plane, and I would disagree that there are no moral or ethics needed because there are no laws or rules that are direct.
As Above, So Below-
coming from my own experiences, I have had intimate relationships on the astral, and built longterm committed relationships with spirits that benefit both parties in spiritual growth and development.
Sexual energy can be used to enhance creative abilities and channelled into other areas besides the genitals.
We as energy cultivating practitioners should take responsibility for what we do on any level of our beings because it does affect the Universal fabric.
I have also had an experience in my earlier energy working days, where an uninvited spirit kept on arousing my sexual energies, and because I didnt conform to its lustings, I was psychically attacked to the point where I had bruises appear on my arms and legs overnight. It took a while to educate and get this spirit to leave me alone.
As I said before As above, so below, if we really are to be spiritually minded and practiced, then it should also comply to whatever plane or dimension we choose to travel to and interact energetically with.
For me there is no point in being spiritual on one plane and unspiritual on another just because I can.
The Universal spirit will not judge us for what we do on any plane, but that does not mean we should be ignorant or disrespectful.
Each to their own cultivation and experiences though...

A book on this topic is Sexual Alchemy, intercourse with spirits by Donald Tyson.