Ok brother. There's a difference between willful sin and uh, ignorant sin. If one has never received Christ and asked forgiveness for their sins, and they are, say...living with their girlfriend and are unmarried. Even if they are being loyal to her and honor the compact of fidelity, they're still sinning, but may not realize (through ignorance) that it is in fact a sin unto God. Once they receive the Lord into their heart and are forgiven and born again (they have a bigger responsibility to obey the Lord). The Lords love covers a multitude of sin, this is true, but as one progresses in their walk (and should be studying and learning/growing), they should be cleaning their life up. At some point, they will learn that they are to be married or it is sin, and if they choose to not get married still...then they've fallen into willful sin and are in effect choosing to put their trust into themselves rather than the Christ. The Lords grace and love is mighty leinient , but at some point...the Lord will consider them mature enough to obey His commandments...and he will have this sin counted against him. We are to obey the Lords laws, precepts and commandments even moreso than mans laws.
Man is steeped in sin from birth pretty much. If we say we have no sin, even after being born again, we are liars. It's important to claim the blood of Jesus upon our life everyday. To recognize our sin and to repent of it and ask forgiveness. If we do this, it is as if we have no sin. We are walking in Christ and growing. But there's no loopholes, God will not be mocked. We can't think to ourselves, ok I'll go party this weekend and have fun and get with a girl and ask forgiveness Sunday morning in church. That's like playing russian roulette and willful sin.
That's why scripture says it better for a man to have never known the Lord, than to learn righteousness and then turn back to his old ways (2 Peter 2:21)