What you are really asking is this, “Will God accept and reward the service of a Christian who is living in sin, and will this sinful lifestyle affect their salvation?” The answer to both questions is NO.

First all, having a sexual relationship with someone outside of marriage is a sin called SEXUAL IMMORALITY (i.e. FORNICATION), even though it is consensual. Let’s read what the Apostle Paul said about this to the saints at Corinth, “Now THE BODY IS NOT FOR SEXUAL IMMORALITY but for the Lord, and the Lord is for the body….Now God has not only raised the Lord, but will also raise us up through His power. Do you not know that your bodies are members of Christ? Shall I then take away the members of Christ and make them members of a prostitute? May it never be! Or do you not know that one who joins himself to a prostitute is one body with her? For He says, ‘THE TWO SHALL BECOME ONE FLESH.’ But the one who joins himself to the Lord is one spirit with Him. FLEE IMMORALITY. Every other sin that a man does is outside the body, but the immoral man sins against his own body. Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? For you were bought with a price; therefore glorify God in your body” (1st Corinthians 6:13, 15-18 NASB).

These words are, for the most part, self-explanatory. They teach us that the Christian’s body belongs to the Lord, who purchased it at a very high price, even His blood (see Acts 20:28 and 1st Peter 1:18-19). Our body is also the “temple of the Holy Spirit,” for the Holy Spirit lives inside of us (see John 14:16-17). In view of these truths, we are NOT to commit sexual immorality; rather we are to FLEE IMMORALITY! We are to keep our bodies pure and in doing so we will “glorify God in our body.” In light of these solemn truths God is NOT glorified if we are living in a sexual relationship outside of marriage; in fact, He considers it a gross sin that is worthy of judgment. “Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for FORNICATIORS AND ADULTERERS GOD WILL JUDGE” (Hebrews 13:4)

In light of this truth it is safe to say that “God will NOT accept the service of a Christian who is living in a consensual sexual relationship.” When that believer stands before the Judgment Seat of Christ (to have his service judged by the Lord), any service he performed while living in sexual immorality will be rejected by the Lord Jesus. We read of this in 1st Corinthians 3:13-15, “Each man’s work will become evident; for the day will show it because it is to be revealed with fire, and the fire itself will test the quality of each man’s work. If any man’s work which he has built on it remains, he will receive a reward. If any man’s work is burned up, he will suffer loss; but he himself will be saved, yet so as through fire.”

In regard to your second question, the last line of 1 Corinthians 3:15 answers your question. One’s service may be BURNED UP (which means it is rejected and the believer suffers the loss of a reward), but HE HIMSELF WILL BE SAVED (which means his salvation is not in question, for his salvation is based on his faith in Christ’s finished work on the cross and not on his service for the Lord). (321.3) (DO)