Matthew 5:27 & 28
You have heard that it was said, "You shall not commit adultery." But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart.
This is another challenging one, because -- If God is going to getcha for this, then I, and every other straight man I know will be going straight to Hell. I really don't believe that's on the program, but then what is this doing in here?
Neither Mark nor Luke mention this saying. The story called 'The Woman Taken In Adultery' could be a later interpretation of it, because -- aside from being thoroughly in character, that story doesn't seem to have been part of any of the known Gospels; while it provides the only place in the tradition where Jesus is said to write anything whatsoever.
The absence of a saying in other Gospels could mean anything or nothing. This particular saying looks like it could be 'Matthew's' elaboration of Jesus' emphasis on inner righteousness above mere observance of external rules.
All the Gospel authors would have participated in groups where what people remembered of the tradition would have frequently been recited; and the material in those recitations would have been 'the same' in general while also being 'different': depending on local conditions and on who wanted to emphasize what on any particular occasion. No particular rendition was the original, 'real' gospel. There would have been overall agreement on what belonged to it but -- until it was eventually written in some form -- room for elaboration within that agreement.
Whether or not this saying goes back to The Guy -- What should it mean?
Everyone likes to see other people, to look at other people, whether or not those people appeal to us sexually: young women, young men, old people, children. God didn't make people good-looking just so we could turn that pleasure into something dumb & ugly.
People whose presence pleasantly twangs our sexual feelings, well -- That's even better! -- except. Except that, (for any of a long list of reasons) following through physically on those feelings would in most cases be a really bad idea.
I think (and prefer to think!) that Jesus is saying, as in the 'Woman Taken' story, that we've got no business condemning anyone for responding to such feelings foolishly, because any of us might have done that (and are lucky if we didn't!)
At the risk of potential inconsistency with the following passage, I'm going to say that we aren't being told to go around with opaque bags over our heads, not being told to suppress sexual feelings for anyone but our spice... It doesn't do to cling to such feelings, to seek them out or dwell on them, but that is another matter.
Given that God can give us anything we ask for... Isn't it a bad idea to nurse desires that would really screw things up if they materialized?