Paul is saying that marriage is a better choice then fornification. This does not mean he endorses adultery. Both Paul and Jesus teach that it is better to remain single if you can control yourself and remain celibate. If you can not control yourself then marriage is a better choice then fornicaton or sex outside of marriage.
Matthew 19
10 His disciples said to Him, “If such is the case of the man with
his wife, it is better not to marry.”
Jesus Teaches on Celibacy
11 But He said to them, “All cannot accept this saying, but only
those to whom it has been given: 12 For there are eunuchs who were born thus from
their mother’s womb, and there are eunuchs who were made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the kingdom of heaven’s sake. He who is able to accept
it, let him accept
it.”
1 cor 7
32 I want you to be free from concern. The unmarried man is concerned about the work of the Lord, how he can please the Lord. 33 But the married man is concerned about the affairs of this world, how he can please his wife, 34 and his interests are divided. The unmarried woman or virgin is concerned about the work of the Lord, how she can be holy in both body and spirit. But the married woman is concerned about the affairs of this world, how she can please her husband.…