It's inferred.
And the LORD God said, “It is not good that man should be alone; I will make him a helper comparable to him.” -Gen 2:18
He who finds a wife finds a good thing,
And obtains favor from the LORD. -Pro 18:22
Whatever the "present distress" was that Paul was talking about, it must not have always been around, and might no longer be. Personally I suspect it was severe persecution. In that case, I would also recommend that people remain single because if your wife gets martyred you're going to have serious problems instead of being only concerned about pleasing God when it's your turn. Still, if anyone wants to be single I don't think that's a mandate to marry.