Separated individuals are still married regardless of the reason for their split. Therefore, they would be guilty of adultery by engaging in romantic relationships or related activities while legally married.
Additionally, marriage is a life-long, binding covenant with only two biblical allowances for divorce: abandonment by an unsaved spouse or adultery (see 1 Corinthians 7:12-15, Matthew 5:31-32, and Matthew 19:8-9).
Considering this fact, people who were abandoned by their spouse or betrayed through infidelity but have not been legally divorced are still married in every sense of the word. They must follow “the law of the land” in their state or country to become officially unmarried before they can date without sinning (see Romans 13:1-7 and 1 Peter 2:13-17).
On the flip side, people who have been legally divorced for reasons not permitted in Scripture are still married regardless of their "official" marital status. Therefore, they would commit adultery by engaging in romantic activities or relationships with someone other than their spouse.
Here are a few additional Scriptures to consider on this very important topic:
Ephesians 5:3-7—But immorality or any impurity or greed must not even be named among you, as is proper among saints; and there must be no filthiness and silly talk, or coarse jesting, which are not fitting, but rather giving of thanks. For this you know with certainty, that no immoral or impure person or covetous man, who is an idolater, has an inheritance in the kingdom of Christ and God.
1 Thessalonians 4:3-8—For this is the will of God, your sanctification; that is, that you abstain from sexual immorality; that each of you know how to possess his own vessel in sanctification and honor, not in lustful passion, like the Gentiles who do not know God; and that no man transgress and defraud his brother in the matter because the Lord is the avenger in all these things, just as we also told you before and solemnly warned you. For God has not called us for the purpose of impurity, but in sanctification. So, he who rejects this is not rejecting man but the God who gives His Holy Spirit to you.
Colossians 3:5-7—Therefore consider the members of your earthly body as dead to immorality, impurity, passion, evil desire, and greed, which amounts to idolatry. For it is because of these things that the wrath of God will come upon the sons of disobedience, and in them you also once walked, when you were living in them.
Hebrews 13:4—Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge.
Romans 12:1-2—Therefore I urge you, brethren, by the mercies of God, to present your bodies a living and holy sacrifice, acceptable to God, which is your spiritual service of worship. And do not be conformed to this world, but be transformed by the renewing of your mind, so that you may prove what the will of God is, that which is good and acceptable and perfect.
I hope this information helped answer whether it is okay to date when separated.
