To pose the question of what causes homosexuality is actually a judgment about homosexuals. If you ask for the causes of homosexuality you assume that this is a disorder which needs to be explained.
It is better to find out what influences and processes that determine sexual preferences, whether this is homosexuality, heterosexuality or something else.
Research today is dedicated to find influences and processes which determine a persons sexual preference. Sexual preference is assumed to be determined by physical (genes), psychological and social factors (environment).
Physical causes for the development of a sexual preference could be certain brain structures or hormones and research is now being done on a certain homo gene. Psychological causes and social factors could play a role. However, not enough is known about these causes for sexual preferences and there hasn't been enough research to say anything reliable about this.