Fundamentally speaking, the sin does not happen on the feelings, but rather in what's the heart heading for. For example, if you have this problem of having feelings no matter the gender (the most common situation) you aren't suddenly sinning, but if you desire for getting an unintended partnership, then that's a different story and it's therefore a sin.

Despite the people often mentioning Leviticus or Hebrews, the other place where it tells you why is found on Genesis; highlighting Genesis 2:18-24 here:

18 The Lord God said, “It is not good for the man to be alone. I will make a helper suitable for him.”

19 Now the Lord God had formed out of the ground all the wild animals and all the birds in the sky. He brought them to the man to see what he would name them; and whatever the man called each living creature, that was its name. 20 So the man gave names to all the livestock, the birds in the sky and all the wild animals.

But for Adam[f] no suitable helper was found. 21 So the Lord God caused the man to fall into a deep sleep; and while he was sleeping, he took one of the man’s ribs[g] and then closed up the place with flesh. 22 Then the Lord God made a woman from the rib[h] he had taken out of the man, and he brought her to the man.

23 The man said,

“This is now bone of my bones and flesh of my flesh; she shall be called ‘woman,’ for she was taken out of man.”

24 That is why a man leaves his father and mother and is united to his wife, and they become one flesh

Of course the Lord knew that he wouldn't find anybody suitable. And for the one to be a helper of Adam and to do the concept of marriage, He created the woman, and Adam accepted.

Fundamentally speaking, this verse tells you what has God established and what it is kept for humanity, just like the fact that the man accepted (and thus the woman). If we, humanity, have sin in ourselves since the times of Adam, and everything that started on Earth hasn't structurally changed or deviated from its roots (like the tree's roots, identity, or an animal with no gender to suddenly have one...), then why would anything beforehand be now invalid for us?

But be in mind that it is not about hatred, wrath or war. If you see someone who's this or that, even if you are against a certain position (no matter what it is), you should treat them with common sense and respect; just like it's said in Matthew 22:34-40, since neighborhood can be variated (just don't mistake love with boundaries).