According to Timothy Kieswetter*, a South African writer on intimacy and Christian living, this is a radical text:
Matt 5:28 “whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart”.
But it is not radical in the way you think. It is not radical because of the punishment in the next 2 verses. (He explains the context of the verse further here.) It is radical because it was said in a society that only punished women for adultery – male adultery was not seen as wrong.** Remember the story of the woman caught in adultery in John 8.
But Jesus says that male adultery is very wrong. In some groups today, what Jesus did would be called white knighting: He came to the rescue of the woman in John 8, while telling the men in Matt 5:28 of the punishment they deserve.
In the real world, it probably meant that Jesus had the same opinion of male and female sin. After all, he did imply, in John 8, that the men could not start throwing stones because they had blame too. But in a world that pooh-poohed male sin and came down like a ton of bricks (literally) on female sin, expressing his heart for everyone meant reminding everyone that male sin is serious, and reminding everyone to be more gentle on female sin.
PS, other topic: If you come from a patriarchal background, you may have also heard that God punished Michal because of how she treated David. Here is another perspective on that.
* The idea for this article is from Timothy Kieswetter in Jesus, die groot vegter vir vroueregte.
** Lev 20:10 shows that this discrimination against women was never from God, the Old Testament also gives the same punishment to both.