Gabe7 said:
He probably knows how you feel about it
Most kids know how their parents feel about issues from the time they are old enough to talk.
You have heard mixed viewpoints here, as would be expected from an open forum.
It is important to look to God and scriptures when making a decision that you expect to be scriptural and set within God's favor. There are many books and media preachers who teach firmly and resolutely -- but when it comes down to living with God as our source, even the influence of professionals is not necessarily the right answer. We each need to choose our own wise steps before God.
There are stories like Jesus' protectiveness in John 8, where a woman was caught in adultery.
And a verse where a decision was made after some time passed:
Acts 21:25 (and javascript:void('null');Acts 15)
"But concerning the Gentiles who have believed, we wrote, having decided that they should abstain from meat sacrificed to idols and from blood and from what is strangled and from fornication."
There are horrible reports in the Bible like Tamar raped by her brother Amnon, with their father angry in response-- but treating the son more like an annoyance than an accountable criminal. (As you know, OT stories like this do not provide commentary -- they are reports of what happened, not how things should be handled.)
1 Timothy 1:10 calls it "contrary to sound doctrine"
1 Corinthians 6:9 shows it as a block to the kingdom of God
In
Ruth 3, Ruth slept all night at the feet of Boaz on the threshing floor.
But the focal point of the situation you face, is two-fold:
1. What terms you declare when a guest stays at your house
2. How you relate to an adult son who has been legally and spiritually responsible for all his decisions for over ten years.
Being in favor with God is not based on one person limiting the actions of another person. Jesus said that adultery in the heart is also a sin... so whether you're able to control the expression of his thoughts or not, does not quite make a difference in the eyes of God.
If you were preventing a murder or another offense against another person, that is a different matter. He is with his girlfriend by consent.
But I do think that any guests should be respectful of their hosts enough to not put them in an uncomfortable position. If a person can't restrain themselves for a day or two, they should get a hotel room. He is an adult making his own decision, but he is now a guest in your home. What he does outside your home is his decision.
He already knows much about the faith, and about your standards. A parent raises their children to make wise decisions toward successful independence. In doing so, they also need to allow for some trial and error... some falling out of the nest and flapping wings without soaring yet. Flying doesn't come by adult birds carrying the fledgling on their backs.