Clarification:
What part exactly does Paul say is a sin?
Love,
O2
In 1 Cor. 7:1-6 Paul gives counsel. It is godly counsel, and does not override, but remains within God's first commandment to humankind which after creation is pronounced "very good," God said, "Be fruitful and multiply." He then reiterated that commandmevtto Noah and his sons: When did Abraham and Sarah cease to cohabit in marriage? When she died. Even then, Abraham remarried and kept fulfilling the commandment.
What other purpose does physical relationship in marriage have" Comforting one's marriage partner. Rebekah comforted Isaac after his mother's death, David comforted Bathsheba after their son's death. There is a joy of body, soul, and spirit in union with one's mate. Denying this is a form of rejection, as physical denial, seed-spilling, or contraception is as murder of the generation to replace us, and a reproof to God for giving us the responsibility of having and raising a godly generation.
In Hebrews, Paul warns not to defile the marriage bed, one defilement of which would be refusal to enjoy, and/or comfort, and/or reproduce.
Paul says celibacy without marriage is OK. Willful celibacy in the marriage bond is not, although it could be that subsequent to confirming the lifetime vows, some physical or medical exigency might require it.
That's how I read the context of the Bible, not just Paul guided by the Holy Spirit.