They almost are the same thing. But obviously people prefer "be going to do" in common conversations. "Be to" sounds a little bit more poetic, preferred by the scholars, writing some papers or philosophical stuff. Grammatically, the "be to" structure implies an "assuming" situation, like the "I would kill myself if I were you" thing. So this "be to" normally would be applied for a "future" assumption (that is almost impossible to happen for real). eg: I would definitely kill you if we were to marry in the future because I don't like you. Oh another situation would be: That is to say, which is to mean... I guess you understand this perfectly, eh?

"we are going to be married in June" conveys the idea of your intention, meaning, it's your plan or idea to get married on a particular date. "we are to be married in June" is simply telling the idea of what your status will become comes June. although both may carry the same expression and intent, the second sentence to me is pretty much exciting to tell. :)

Your question is wrong. It should be the difference between: 1. be to be (married) 2. be going to be (married) The two expressions: 1. be to be + past participle 2. be going to be + past participle are the same in meaning.

I think in first almost to be is ommited, like marriage process is almost finished. Second means you are planing.