Is ‘living in sin’ a sin? Is marriage the only way to have a sexual relationship? Depends on your social circles. I got excluded from a great church because of ‘living in sin’ (plus divorce was another factor), but love won the day. Marriage is a social circle phenomenon. Left to your private decision for you and your partner or potential partner you might simply make a private commitment as my partner and I did. Later I bowed to social pressure and got married formally when my partner accepted. But only after I lived together ‘in sin’ for years. Personally I don’t believe it was sin. Singleness with living in lustfulness is a sin though. The church did not chuck me out for that. So sin is a factor but not the only factor. Social acceptability is a factor too.
