The Divorced to Remain Unmarried
1Cor 7:10-11 To the married I give this command (not I, but the Lord): A wife must not separate from her husband. But if she does, she must remain unmarried or else be reconciled to her husband. And a husband must not divorce his wife.
For a wife to enter into an "unmarried" state there must have been a divorce. So by "separate" he's referring to divorce. But actually the Biblical view of divorced is no different than that of a separation, seeing as divorce doesn't nullify a marriage. And we can see this fact here as this "unmarried:" women is obligated to stay unmarried. Why? Because the divorce didn't nullify the marriage. "By law a married woman is bound to her husband as long as he is alive" Romans 7:2a Thus Jesus said, "Anyone who divorces his wife and marries another woman commits adultery, and the man who marries a divorced woman commits adultery." Luke 16:18 Or in the case of the wife, "if she divorces her husband and marries another man, she commits adultery." Mk 10:12
Notice also that this unmarried woman is spoken of as having a husband! For another example of this note God's divorce. "I gave faithless Israel her certificate of divorce and sent her away because of all her adulteries." Jer 3:8 and yet a few verses later he says, "Return, faithless people," declares the LORD, "for I am your husband." Jer 3:14 The divorced should be open to reconciliation with their spouse, though that is not the reason why one is restricted from marrying someone else.
And just to briefly deal with the exception clause found in Matthew which says, "except for sexual immorality" Mat 5:32b, it's likely referring to illegitimate marriages, such as the case when one divorces their spouse and marries another, which Jesus declared to be adultery and thus an illegitimate marriage. To divorce out of illegitimate marriages does not obligate one to remain unmarried. And likewise with other sexually immoral marriages, like homosexual marriages or incestuous marriages or the like.