What is an apostasy, from the George Bernard Shaw play, Man and Superman?

An apostasy is an abandoning of what you've believed in, whether it's faith, a cause, or principles. This falling away may reflect a simple change of heart or a serious desertion.

In George Bernard Shaw's play, Man and Superman, bachelor Jack Tanner feels that he is being forced into marriage and sees himself as a condemned man who has no control over his fate. He explosively denounces marriage:

Marriage is to me apostasy, profanation of the sanctuary of my soul, violation of my manhood, sale of my birthright, shameful surrender, ignominious capitulation, acceptance of defeat. I shall decay like a thing that has served its purpose and is done with; I shall change from a man with a future to a man with a past . . .