Ed Miliband, the leader of the Labour Party in the UK, did not marry until he was 42.  Why?  Because he lived with his girl friend for about 10 years without getting married.  Other men may not get married for the same reason.  Some do not marry because they were busy caring for a sick parent.  Or their career was too turbulent to marry.  If you are serious and he is serious in your correspondence, then it is absolutely fine to ask "Why did you never marry?"  He will be accustomed to people asking this question and will explain.  If he is offended by this question, move on to another man.  If his answer is not good, move on to another man.  If he says that he was gay and now wants to try women, you can move on to another man.  But on balance, there is no problem with asking the man straight out.