Please think if the same person by chance made you pregnant and you delivered a baby and left you, would you leave him simply as you expect him to be? what difference makes your activity than to that? you make two men life screwed and later if your husband comes to know think of your situation, in society you will be named different.
Best, convince your parents and apprise the situation to your live-in man and marry him
second if not possible that, tell the person wh you marry honestly, if he is broad minded ( I dont think he will agree), then marriage is best.
otherwise, you choose hell to live