If someone says they are living God's plan for them but this includes a sinful relationship
with a boyfriend or girlfriend, including having premarital sex, can that be God's
plan for that person?
First, let me say, 'living God's plan' as you put it is something no one but God knows anything about. However, I believe God's plan would never include anything He repeatedly tells us in His word, is sinful and an abomination to Him. This includes a ‘sinful relationship with a boyfriend or girlfriend’. Living together or co-habitating with one another is included in this. There are many verses in the Bible that tells us that premarital sex is wrong and we are not to have anything to do with it;
"But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints;" Eph. 5:3
"For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who
do not know God;" 1Thes. 4:3-5
The Bible states, sex is for married couples. Not for personal gratification. We need to remember, that God created the sacred relationship of marriage for our benefit. He knows, that the intimacy in a relationship between a husband and a wife, brings their hearts and souls together in a wonderful way. To have a family unit that is united in Christ is the intent. Not a one 'night stand', or a relationship that can die in a month or two which only results in pain and suffering. When we give ourselves to each other, truly our hearts and minds, there is peace and a sense of well being which God intended. This is what He has always intended as shown when He made Eve, a woman for Adam. It was since this first act of God that the intention was to be for a family to be created, to produce offspring and live a life in union with Him.
God also knew of the dangers of having sex with just anyone. Look at the diseases that have come to fruition. One of the causes of this is because of the 'sex with anyone' mentality. This is not the way He intended for an intimate, wholesome relationship to be like. Marriage is to be an honorable, sincere relationship, and He will not tolerate anything less. Fornication (pre-marital sex) is one of the sins that will keep us from our inheritance. The ones who believe they have God's grace when they sin, will ultimately know the truth;
"Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge." Heb. 13:4