Sacramentally married? No.
Becoming one flesh? Yes.
St. Paul is remarking on the reality of sex. Even if you’re not married to the person you first have sex with, you’ll always be united with them in body. Marriage really gives couples the graces that rightfully accompany sex. Without those graces, without God’s “official stamp of approval” you could say, sex is misused, and misused seriously.
In context, (reading all of that chapter) St. Paul is also remarking on the sacredness of our bodies, as temples of the Holy Spirit, and for many of us, weekly having within us the True Presence. We must protect our bodies, especially in regards to sexual impurities.