1 Corinthians 7:6–7
6 But I speak this by permission, and not of commandment.
7 For I would that all men were even as I myself. But every man hath his proper gift of God, one after this manner, and another after that.
8 I say therefore to the unmarried and widows, It is good for them if they abide even as I.
9 But if they cannot contain, let them marry: for it is better to marry than to burn.
What Paul has just written to the Corinthians is not of commandment. Yet, that does not make it any less accurate, and it does not negate the fact that what Paul wrote is inspired by God. He is not making it up on his own. He has received permission to make the statements. But, it is not a command from the Lord that if sexual relations are withheld for the wrong reasons that it is a sin.
It is probably not wise to withhold sexual relations since it could open the door for one spouse to give in to sexual temptations due to lack of self-control. If you want a good marriage, make sure there is good sexuality within the marriage. Likewise, if we want a good society, there needs to be a healthy understanding of sexual relations within that society.
The Lord gave Paul permission to share about sexual relations within marriage, now Paul gives a wish for every man. His desire: I would that all men were even as I. Paul was single–but he had the spiritual gift of singleness. Everyone has a proper spiritual gift, one after this manner, and another after that. How would one know if he or she had the gift of singleness? It would have to gauged by their self-control or lack of self-control in sexual issues.
Some say that maybe Paul had been married once and was now a widower since he speaks to the unmarried and widows, that if they have the gift of singleness like he does, it is good.
But if they cannot contain, in other words, constrain themselves in sexual matters, get married: for it is better to marry than to burn. The context is not talking about burning in hell. All sin is forgivable so that cannot be the meaning of the word. It is a metaphor. In context, he means to get married, so you don’t burn with lustful sexual desires.