Style over substenance. An awful, pretentious, bullying style that is transparently attempts to intimidate the opponent through grandiose and obscure words and blatant dominance behaviour hiding behind the most threadbare pretence of intellectualism.
Fundamentally, your argument is a false dichotomy between reproductive and recreational sex.
In humans, sex serves not only the purpose of reproduction, but also very importantly a secondary social purpose of bonding, which makes a lot of sense for a species with such intense long-term investment in its offspring. This is clearly an “intended” application rather than an incidental one: The human sex drive is retained even outside of the female’s fertile window and indeed after menopause, and indeed, during pregnancy, where further impregnation is not possible, there frequently are phases of increased libido. And, most relevantly, in both males and females, there is an elaborate system of escalating stimulation involving many erogenous zones that are not even close to the outer genitalia. In a healthy relationship, sex is generally a complex drawn-out act involving various forms of sexual stimulation - and only through this, both bodies are really ready for copulation. Consequently, your distinction between “sex” and “masturbation” can in many cases only be determined retroactively, on whether somewhere down the line towards the end of the session, coitus occurs.
The reason why sex should and must be defined as the reproductive act is that anything else is illogical, insane, and, frankly, unhealthy. Humans have allowed new venereal diseases to spread in the modern generation which were unknown in the ancient world, since the pursuit of perverse sexual pleasure, involving the abuse of sexual organs by thrusting them into orifices where nature never intended them to do, is unhealthy, and spreads disease.
And here, you are conflating sexual pleasure with promiscuity. It is very much possible to have strong intense sex with foreplay within a monogamous marriage. Also, I am not an expert on venereal diseases, but I am not aware of any that has its origin in germs displaced to the vagina from other orifices. Furthermore, coitus is very much capable of sexual transmission as well. Finally, the only major venereal diseases that were “unknown to the ancient world” are Syphilis, a disease that is likely native to the New World and consequently only spread to Europe with Columbus (and from then on, it was a widespread if swept-under-the-rug problem throughout Europe until the 20th Century - so much for the efficiency of Christian sexual repression!) and AIDS, a relatively recent zoonosis (which is a systemic infection affecting all bodily fluids, so it’s far more likely to have been introduced through bushmeat than through bestiality).