Jesus’ great commandment is: “Love one another as I have loved you.” His discourse goes on to show that, ultimately, the love he has in mind is that which culminates, like his did, in laying down your life for those you love. So we are not talking emotion, romance or sex (sometimes called “making love”.) We are talking self-giving, maximum focus on the other rather than on oneself. The opposite of love is hatred. So that, if the great commandment is to love, the great sin is hatred. When we hate we want to eliminate the object of our hate. If we can get away from the object of our hate, we might, but should avoidance be too diffult or impossible, hate pushes us to destruction. We see hatred when we see war, aggression and violence.
Thinking in terms of polarity, with love one pole and hatred the other, I think we can see that sex is way closer to the love pole than the hatred pole, if, indeed, sex can be coupled with hatred at all. That tells me that sex is not really connected to sin, the opposite of love, at all. A surprising conclusion when we have been brought in a society and culture where the primary sin was, de facto, sex.
The reasons for our hatred, the cause of our hatred, can be varied and very subjective. We may hate something or somebody and want to eliminate the object of our hate because we perceive that object as, somehow, a threat to our existence, ideals or projects, whether the object is, in reality, a threat or not. So we can observe that a lot of the hatred in the world is irrational, and so, we can say, more tainted by sin because it is in opposition to truth, which is, like love, part of the nature of God.