-FoC-

Firstly that commandment is given to a marriage where BOTH are believers as proven by the very next verse where the UNequally yoked are given instruction about their marriages and what the situation is in the case of abandonment.

Secondly, Jesus' exception is still be be harmonized with Pauls own words as well.
Paul is answering some questions that he apparently had been asked by the Corinthians, and clearly some of those questions must have been about whether they could just up and leave their spouses for particular reasons.
Neither Paul nor Christs intent was to doom someone to celebacy that they couldnt handle. They both, as Moses in Deut 24:!-4, were trying to curb frivolous divorce and cause people to start taking marriage a bit more seriously.