Original: Common reasons for practicing sexual abstinence include: religious or philosophical reasons material reasons (to prevent conception - undesired pregnancy - or sexually transmitted disease transmission); psycho-sociological reasons (e.g., clinical depression, social anxiety disorder, increasing testosterone in males, or negative past experiences); or, legal injunctions requiring conformity.
Simplified: Common reasons to deliberately abstain from the physical expression of sexual desire include religious or philosophical reasons (e.g. chastity), material reasons (to prevent conception (undesired pregnancy) or Sexually Transmitted Disease transmission), or to conform to legal regulations.