A man and a woman living together before marriage is lasciviousness and is SIN. Experimenting sex and checking for compatibility even among an engaged couple is sinful - there is no guarantee the marriage will happen at all. Living together before marriage is not biblical and is sexual immorality and plain harlotry and a sin. In the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils, forbidding to marry, 1Tim 4:1, 3.