Hi there,
Ok I'm late to this but here's what I've found...
Firstly, you're right, Matthew 5:28 has been weaponized. Guilt makes money.
But what Jesus was addressing here was property rights, oathbreaking, and the inward heart.
The key word here is "lust". Today we understand "immoral sexual desire" when we see that word. But this is not what Christ meant. He used the Greek word "Epithumia". It's a neutral word meaning "strong desire". Jesus used it Himself when describing how much He wanted to have supper with the disciples. What's important to note is that this word has an action component to it. Hence why it refers to a strong desire. There is this inward decision one makes when desiring whatever object it may be.
In this case, one may say, "Ok, you're still desiring a woman and thus committing adultery". This is where the property rights comes into play. Jesus was explaining this commandment in light of the 10th commandment. The 10th commandment deals with not coveting one's neighbour's property. Guess what the Greek word is for "covet"... Epithumia. Now the object of desire in Matthew 5:28 is a woman, Greek - "gunh". This can mean woman or wife. But, seeing that Jesus is talking about one's neighbour's property, it can only mean one of two things, a wife, or a virgin still under her father's protection. However, He's speaking about adultery. Adultery is about oathbreaking, in this case, marital unfaithfulness. You cannot be unfaithful with a virgin to my knowledge. So, a correct reading of Matthew 5:28 may sound like "Anyone who fixes their desire upon the wife of another man has already broken the oath within his heart".
If Matthew 5:28 meant that any man looking sexually at any woman was adultery, then polygamy would not be possible. Matthew 5:28 was Jesus addressing the inward wickedness, even though the outward physical side seemed fine. It was a question as to whether one's heart was in the right place.
With much love!