Sexuality
Tchaikovsky had clear same-sex tendencies. Some of the his closest relationships were with men. He sought out the company of other same-sex attracted men in his circle for extended periods, "associating openly and establishing professional connections with them." Relevant portions in his brother Modest's autobiography, tells of the composer's sexual orientation, have been published, as well as letters previously suppressed by Soviet censors in which Tchaikovsky openly writes of his sexual preference.
Marriage
One of his admirers, a Moscow Conservatory student Antonina Ivanovna Milyukova, was constantly writing him love letters. She even threatened to commit suicide if Tchaikovsky didn't marry her. He did marry her, but their brief marriage in the summer of 1877 lasted only a few weeks and caused Tchaikovsky to have a nervous breakdown. He even made a suicide attempt himself, by throwing himself into a river. In September of 1877 Tchaikovsky separated from Milyukova. She eventually ended up in an insane asylum, where she spent over 20 years and died. They never saw each other again, and although their marriage was ended legally, Tchaikovsky continued to support her financially until his own death in 1893.