How does Judaism consider sexual immorality?

Answer 1

The phrasing of the question is confusing. It could be one of two different questions:

1) What things does Judaism consider to be sexual immorality?

• Adultery - e.g. sex between a married individual and someone who is not the spouse of the married individuals
  
• Sex with More Than Two Individuals - e.g. a menage-a-trois or an orgy
  
• Sex Out of Wedlock - e.g. sex between two non-married individuals (this is contested between Orthodox Judaism and Liberal Judaism)
  
• Niddah - e.g. having sex with a with a woman who is on her period.
  
• Religious intermarriage - e.g. a marriage between a Jew and a Non-Jew (Note that marriage between Jews-by-birth and Jews-by-Conversion is permitted for all Jews except Cohens)
• Incestuous relations - e.g. marriages/sex between family members.
  
• Marriages with certain Canaanite Groups (even after conversion)
  
• Mamzer - e.g. a person who is either born of adultery, incest, or has a mamzer as a parent.
• Marriage with Eunuchs
  
• Homosexuality and bisexuality (this is contested between Orthodox Judaism and Liberal Judaism)
  
• Bestiality
• Sex/Marriage with a person unable to give consent (such as a mentally handicapped person)
  

2) What is the proper punishment in Judaism for sexual immorality?
Most forms of sexual immorality in Judaism are subject to capital punishment. However, Judaism has such stringent requirements on the implementation of capital punishment, that excommunication or local banishment are far more common as punishments for sexual immorality. Only the Orthodox Jewish community is tight-knit enough and cares enough about sexual immortality to enforce these excommunications and local banishments.

Answer 2

Judaism forbids sexual immorality (Leviticus ch.18).