Wikipedia provides two good summaries of how the Ten Commandments are viewed:
The Ten Commandments are written with room for varying interpretation, reflecting their role as a summary of fundamental principles. They are not as explicit or as detailed as rules or as many other biblical laws and commandments, because they provide guiding principles that apply universally, across changing circumstances. They do not specify punishments for their violation. Their precise import must be worked out in each separate situation. — Religious interpretations
The Ten Commandments form the basis of Jewish law, stating God's universal and timeless standard of right and wrong – unlike the rest of the 613 commandments in the Torah, which include, for example, various duties and ceremonies such as the kashrut dietary laws, and now unobservable rituals to be performed by priests in the Holy Temple. Jewish tradition considers the Ten Commandments the theological basis for the rest of the commandments. Philo, in his four-book work The Special Laws, treated the Ten Commandments as headings under which he discussed other related commandments. Similarly, in The Decalogue he stated that "under [the "commandment… against adulterers"] many other commands are conveyed by implication, such as that against seducers, that against practisers of unnatural crimes, that against all who live in debauchery, that against all men who indulge in illicit and incontinent connections." Others, such as Rabbi Saadia Gaon, have also made groupings of the commandments according to their links with the Ten Commandments. — Judaism
The key characteristic is that these brief commandments provide the general principles upon which all the other laws are based.
That they don't provide explicit penalties for their violation makes their being guiding principles more obvious.
The civil laws of most western countries are founded upon these principles (the last six anyway).
Jesus provided an even more general principle when he summarized the commandments into two categories, one for the first four and one for the last six:
Then one of them, a lawyer, asked Him a question, testing Him, and saying,
“Teacher, which is the great commandment in the law?”
Jesus said to him, “‘You shall love the LORD your God with all your heart, with all your soul, and with all your mind.’
This is the first and great commandment.
“And the second is like it: ‘You shall love your neighbor as yourself.’
“On these two commandments hang all the Law and the Prophets.”
— Matthew 22:35–40 (NKJV)
He wasn't doing away with all the laws and replacing them with two; he was summarizing all laws into two guiding principles.
The seventh commandment against adultery supports marital fidelity and forbids sexual impropriety.
Any sexual relationship that isn't between a married man and woman violates this principle.