The Church has always considered fornication (sexual activity between singles) and adultery (sexual activity where at least one person is married, betrothed, or romantically bound to another). JKRH makes a good point about how most people in Jesus’ time were bound to another person in a more-than-friends way.
However, just because sexual activity between singles is not technically adultery, doesn’t make it right. Jesus’ point was that looking at someone in lust is misusing their body. A person who does that cares only about the physical features of the other person, not their soul. That doesn’t mean it’s wrong to find someone attractive, but it is wrong to treat their body like an object for pleasure and pleasuring. It is the person that should be important. Jesus even says that using “the legalities of marriage as a cover for lust is adultery”. Why would it be adultery to get married before having sex? Because in this case, the main reason the couple are getting married is to have sex (to satisfy their lust), rather than because they want to knit their souls together, in which case sexual desire for their partner is just an added bonus.