Why are you torturing yourself? It's really okay to be monogamous, as long as you're being honest with yourself and your partner. You have the right to live your life in a manner that's best for you. It pushes my buttons when someone passes judgement on you for not living the same way they do.
If your partner wants to be poly, it's his responsibility to talk to you about it. If it frightens you that he wants to be poly and isn't saying something, then talk to him about it. If it helps, write down what you want to say. There is no harm in saying, "I'm sorry, but I'm not polyamorous." It's okay to say you're not comfortable with something. But after a year and a half, it seems likely to me that he's just fine with monogamy and was probably just bringing up something that was interesting, and possibly was curious to see how you would react to it. As human beings, we do this kind of thing all the time. "Did you hear about the guy who...? What do you think of that?"
As for your questions:
- Poly people find partners in a wide variety of ways, much the same way as mono people. I think Ariakas covered this well.
- You said, "Doesn't starting a dialogue about the poly lifestyle often result in a "sorry, but I couldn't live that way" ending?" Sometimes. If one partner can't live with poly, it's up to the two of them to figure out if the relationship has to end, if one partner can live without poly, or if there's some other compromise they can come up with.
- You asked, "Do mono people sometimes end up in a poly or "V" relationship simply so they won't lose the partner they adore?" Sometimes, though I would hope that everyone would agree on a compromise and that you would be true to yourself.
- You asked, "If a hitherto mono person spoke to their mono partner about potentially opening up the relationship and got a bad reaction from them, is there any going back? Will that person always have an interest in polyamory from then on, or can things go back to the way they were?" Let's get something straight--polyamory is not like air. Not everyone needs it. Having an interest in it does not necessarily mean they have to have it. I've seen people state that they need poly in their lives and that they can not be involved in a monogamous relationship. I've also seen poly people decide to be monogamous because they want the monogamous partner more than they want poly. I think that it would be far better to be honest and find out up front what's going on than to think that not talking about it will prevent your partner from being poly.
As for never ever changing your feelings about poly...that might be. I know Mono feels he is hardwired for monogamy and won't change. I know that I was exposed to poly 20 years ago by a friend who judged me constantly for not living my life the way he was. I got a very negative view of poly and told my husband that if he wanted to see other people he was welcome to do so the second the ink dried on the divorce paperwork. I shake my head constantly and kind of laugh at the trick life has played on me...I was the one who made the decision to be poly, after we met the right people. So who knows? But if you are mono for life, you certainly don't have to feel ashamed of it.