Keep Sex Set Apart From The World

KJV (Hebrews 13:4)

Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge

NIV (Hebrews 13:4)

Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral

It's not saying everything is permissible, but that the bed should remain undefiled. That's why it mentions whoremongers and adulterers. The whoremongers reference let's us know that we can't get besides ourselves when having sex. Lasciviousness is going beyond normal sexual desires and that's what whoremongers do. They're what culture calls thirsty people and the Old Testament call wanton. Things like oral sex, anal sex and masturbation all fall under lasciviousness.