How Does the Bible Define Sexual Immorality?

"Let marriage be be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous" -Hebrews 13:4. 

When warning the wicked of the consequences of sexual immorality, we often get a response of, "What's immoral about what I am doing? Everyone else is doing it! The Bible doesn't even define 'sexual immorality.'" Who are you, we are asked, to judge what I do? 

First, let us ask what consequences the Bible warns us will apply to sexual immorality. Is it important enough to worry about it? Here is the Bible's answer: "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God" (I Corinthians 6:9-10). So, the Apostle tells us, the sexually immoral are precluded from the kingdom of God. That is, from eternal life with the godly. No more serious consequence can be imagined! 

So what, then, constitutes sexual immorality? We see it in Hebrews 13:4, quoted at the top. The author does not give us a list of forbidden acts (for such, refer to Leviticus 18). Rather, this is one of the few cases in which we are given one moral answer, leaving all others to be excluded. He explicitly tells us that sexual immorality is that which is outside of or besides the marital relationship. And who can be married? "[Jesus said], have you not read that He who created them from the beginning made them male and female, and said, 'Therefore, a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh'? So they are no longer two, but one flesh" (Matthew 19:5-6). In these two sentences, Jesus tells us that marriage in God's plan consists of one man joined to one woman, precluding any other combinations, regardless of the tides of political or social opinion. 

It has become defined among the politically correct, whether they claim to be Christians or not, that speaking against any "alternative lifestyles" is judgmentalism, and to be excluded from social discourse or the teachings of church leaders. Yet, what can we say of people who refuse to warn people of the consequences of their choices, given what we know from I Corinthians 6? What could be more hateful than to allow the wicked to go in ignorance into an eternity in Hell?