Lust happens involuntarily, thus lust as a sin is a ludicrous concept.
I have been reading and viewing materials on the function of hormones. These have shown me some interesting things.
The study referenced in this article points to lust being non-voluntary and happening automatically. https://www.telegraph.co.uk/news/science/science-news/3347689/Male-lust-is-blind-research-suggests.html
The article in the link states that, when alone in a room with a female, the male participants in the study showed higher levels of testosterone.
Why is this relevant? Because testosterone drives lust. Articles such as: http://sitn.hms.harvard.edu/flash/2017/ ... anionship/ explain some things about this.
Now, while not all Christians would think of lust as a sin, some do and I feel there are verses that point to this. (Quoting from the NIV version as I've gotten a smaller amount of objections to using this version than others in the past.)
Matthew 5:28
28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.
This verse seems to point to lust being equal to adultery, which is spelled out as a sin.
Mark 7:20-23
20 He went on: “What comes out of a person is what defiles them. 21 For it is from within, out of a person’s heart, that evil thoughts come—sexual immorality, theft, murder, 22 adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly. 23 All these evils come from inside and defile a person.”
From within indeed. Perhaps from chemicals of which we have no control. This says evil thoughts can occur, not only actions. There's a case that lust falls under sexual immorality and/or lewdness.
1 Thessalonians 4:3-5
3 It is God’s will that you should be sanctified: that you should avoid sexual immorality; 4 that each of you should learn to control your own body[a] in a way that is holy and honorable, 5 not in passionate lust like the pagans, who do not know God;
Passionate lust is said in these verses to be going against God's will: in other words sin. It's lumped in with sexual immorality. It says to learn to control your body in a way that prevents lust, however one cannot control one's hormones in the blood stream voluntarily.
Edit: I believe there are more verses regarding lust that I could list if I am asked to further back this up.
So, if one agrees to the premise that lust is a sin, one might be guilty of sin due to no voluntary action of one's own. The hormones in your blood stream (including but perhaps not limited to testosterone), would cause one to lust subconsciously, and therefore to sin subconsiously.
I would say that the concept of lust as sin is ludicrous in that one doesn't voluntarily participate in lust. A creator God would have presumably designed us with hormones such as testosterone, thus causing the lust. What purpose could there be for this other than to set us up for failure or to cause us to sin?
It's unreasonable for a being (such as a god) to cause involuntary actions to occur and then proceed to punish individuals for those actions (as sins are punished).