At the basic Christian level it is to regard sex as legitimate only within the context of marriage between a man and a women.
Other types of sex being sinful in the specific sense of requiring repentance.
But that is a negative, exclusionary definition; and I would (from my perspective) go beyond it to state the principles positively.
That is: the highest ideal of human life is marriage and family. The highest ideal of marriage is monogamous. And the Highest ideal of monogamy is an eternal commitment - going beyond death into resurrected life.
Note: "ideal". Since there is no such thing as equality or neutrality - all phenomena must be ranked according to the ideal - and the ideal references this ranking.
The basis of these ideals entails than men and women are qualitatively-different in some ultimate spiritual way and in some divinely-ordained fashion - and not merely as social roles, nor merely as biological contingency.
Thus marriage is a higher ideal than celibacy, monogamy than polygamy - including serial polygamy (multiple marriages for any reason); marriage with children than without; permanent marriage intending an eternal relation than until death.
Now, this is an imperfect world inhabited by imperfect people - and we must, indeed, assume that this is by divine intent and design; since mortal life is for experiencing and learning. So ideals are very often impossible in practice - but that does not stop them being the ideal.
The difference is that when we fall short of ideals we need to repent - and by repent I mean acknowledge the reality of, and divine-intention behind, the ideal.
So I am not talking about what people achieve in life, in adhereing to thse ideals - I am talking about what they regard as ideal, and what they regard as requiring repentance. It is not in terms of behaviour that they are judged - but in terms of their ideals - their motivations.
What is the point of all this? the point is that the sexual revolution is the prime litmus test of our time and place - and the alignment of the modern spiritual warfare is therefore most simply and validly defined in terms of attitude to The Sexual Revolution: that is, insofar as a person's attitude challenges the above ideals.
All those who favour the sexual revolution, in all or any of the ways in contradiction to the ideals - are On The Other Side, overall and in net-effect. Insofar as they try to justify or promote ideals in line with the sexual revolution - they are in a state of unrepented sin, hence deliberately choosing to reject salvation.
That means most-people - That means nearly-everybody.
But such is the nature of this era.