Kerry - If a unmarried person engages in sex before marriage, then they are not keeping "their own future" marriage bed pure, nor are they honoring the institution of marriage, it is sexually immoral.
Then such a person doesn't engage with a married person and simply doesn't get married, thereby eliminating the risk of an 'impure bed'.
You are assuming that "sexually immoral" automatically means all sex outside of marriage. This is debatable based on the Bible itself.
I'm not convinced you're wrong, just convinced that this verse is not saying what you're saying.