Sexual Immorality appears to be the following Four(4): Incest, Male Homosexuality in part but perhaps not in whole i.e. "sex with a man as with a woman", Bestiality (sex with animals), and Adultery i.e. a man having sex with another man's wife.
Fornication would appear to encompass all these too. Rape is listed as a crime, but not sexually immoral unless it violates the above Four(4). Having sex with children is not sexually immoral unless it violates one of the above Four(4) e.g. incest, etc. Yet society has placed restrictions on this by placing an age-of-sexual-consent in the pudding.
To extrapolate... it is deemed ok for
(A) married man to have sex with his wife and a single woman/women either at the same time or separate. e.g. Twosomes, Threesomes, Foursomes, etc.
(B) A single woman could have sex with a married man, a married woman, a single woman, or a single man. But not necessarily at the same time e.g. a married man and a single man together at the same time violates the Fornication Clause. Nor could she have sex with a married man and a married woman at the same time if the married man was not married to the married woman. Etc.
(C) A single man could have sex with a single woman or single women.
(D) A married women could have sex with her husband and/or a single woman either separate or together. However, a married woman having sex with a married woman may violate the Fornication Clause if both had different husbands (not the same husband) since both women are in a marriage covenant with a different man.
See where I am going with this? I have seen Jewish Charts that make this process of elimination much more visual and thus easy to follow. "WHAT'S IN YOUR WALLET?" i.e. don't be caught away from home without your handy, quick reference, to what is sexually moral, for when that opportunity knocks. These laws on sexual immorality can be found in the Bible Old Testament Books of Leviticus and Deuteronomy. Since the Bible explicitly tells us what is sexually immoral, it is up to us as followers of God to deduce from that what is sexually moral, similar to being told not to eat seafood without both fins and scales and it then being our job to determine if a particular fish qualifies as acceptable to eat.