Jamieson, Fausset and Brown on 1 Corinthians 7:2

2. Here the general rule is given

to avoid fornication--More literally, "on account of fornications," to which as being very prevalent at Corinth, and not even counted sins among the heathen, unmarried persons might be tempted. The plural, "fornications," marks irregular lusts, as contrasted with the unity of the marriage relation [Bengel].

let every man have--a positive command to all who have not the gift of continency, in fact to the great majority of the world (1Co 7:5). The dignity of marriage is set forth by Paul (Ep 5:25-32), in the fact that it signifies the mystical union between Christ and the Church.