Is sex okay with someone else if you’re separated from your spouse?
No, sex is not okay with anyone that you are not married to. If you are married to someone, even if you are separated, and you have sexual relations with someone other than your spouse, then you have committed adultery. It makes absolutely no difference if you love that person, if your spouse was abusive, if you don’t love your spouse, or if you are separated. Marriage is marriage and to have relations with anyone other than your spouse is adultery, plain and simple. Matt. 15:19-20, “For out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders. 20 “These are the things which defile the man.” Acts 15:29, “…that you abstain from things sacrificed to idols and from blood and from things strangled and from fornication; if you keep yourselves free from such things, you will do well.” 1 Cor. 6:9-10, “Or do you not know that the unrighteous shall not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, 10 nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, shall inherit the kingdom of God.” Heb. 13:4, “Let marriage be held in honor among all, and let the marriage bed be undefiled; for fornicators and adulterers God will judge.”
“Basic Christian Doctrine” by Matthew Slick, http://www.carm.org/basicdoc.htm.