Actually, I don't believe that's correct.
You have heard the commandment that says, You must not commit adultery.But I say, anyone who even looks at a woman with lust has already committed adultery with her in his heart. So if your eyeeven your good eyecauses you to lust, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell. And if your handeven your stronger handcauses you to sin, cut it off and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell."
--Matthew 5:27-30, NLT
Even heterosexuals thinking about lusting after each other is a sin. It requires forgiveness. Homosexuals lusting after each other is easily as bad. Yet few seriously discuss full-on acceptance of adultery. Homosexuality, however, gets quite a full-court press for acceptance for some reason.
“Lust” is a sin. Looking or being attracted, isn’t.
It's true that there are "internal acts" which are deliberate: indulging in lustful fantasies, using mental images for self-stimulation, generally running porn in your head. To the entent that they are intentional, these are sins in themselves, whether you ever act on them externally or not.
This is different, however, from unbidden temptations: feelings of desire which distract or torment you, and which you did not choose to harbor in your head. People troubled in this way (whether they are temptations to lust, wrath, jealousy, envy, revenge or whatever) have an obligation to figure out how to drain the power out of these thoughts, change them, or escape them without succumbing.
Bottom line: unbidden feelings are not sins, even if they assail you strongly and persistently. Inward surrender to lustful thoughts, though, is a sin.
I reckon 99% of people experience forbidden appetites at some point in their lives -- maybe for years ---- and have to learn how to achieve a level of inward peace and self-control.
This is a major and gradual achievement, a struggle, and not an easy one for most of us.