I'm definitely of the opinion that some people have the capacity to romantically love more than one person, and that others don't. I'm also of the opinion, based on my experiences, that there are actually quite a few people out there that believe they are mono until they run up against something different and CAN in fact open themselves up to a poly lifestyle. My SO came out as poly two years ago and I didn't like the idea at first (I didn't have the benefit of knowing what poly meant or have any kind of forum like this to turn to and boy do I ever wish I had!) and I was pretty upset that my SO could love someone else. Luckily we had a healthy relationship before that and we were able to communicate with each other about my jealousy, her feelings for M (her boyfriend), and all the other complicated issues that came up. We had to learn the hard way, through trial and error, but now at the other end of things I wouldn't go back to being mono if I could. I'm happy in my relationship with K (my SO) and happy with her relationship with M. I've met M, and have tried to be friends with him, but he's pretty uncomfortable with that idea, so unfortunately we don't have the ideal poly relationship, but we've worked out the kinks nonetheless.
My Friend J was in a relationship with a man she was really attracted to for a long time while keeping it very casual/non-committed because she is poly too and was afraid G (her love interest) couldn't accept it. When they finally did have a talk about a more serious relationship, J laid it all out on the table and G reacted how she thought he would, but saying he couldn't deal with it and basically giving up on the relationship. About two weeks after that happened G called up J and asked her if they could talk. Lo and behold, he had thought about it a lot (and missed J terribly) and decided/realized that he could compromise, that while he wanted monogamy he could let J be poly and that they could work it out. G realizes it will be difficult to navigate this kind of relationship, but he loves J and wants to be with her, so he's willing to give it a try (Yay!).
I recently met a woman on a business trip/conference type thing, and while I did not tell her I was in a relationship at first, I don't think I did anything ethically wrong. She let her interest be known (namely by hanging out with me at every spare moment and being very friendly) and I tried to be friendly and polite without giving any signs that I was romantically interested (which I'm guessing I probably failed at, I'm a bit of an open book). So after about a week of this (which boosted my ego rather out of proportion) I came right out and told her that I really liked her, and I told her about everything else, my relationship with K and her relationship with M and that I wasn't going to leave K under any circumstances but I still wanted to see her. I expected that would be the end of it right there, but luckily (I guess, I can't make myself feel too pleased with how things are turning out) we had a very long conversation, her being very much in tune with the mono philosophy, me being aligned with a poly philosophy. I'm still not sure I convinced her that I don't want to leave K, I don't think I really convinced her that a person can love more than one person at the same time, but when we both had to go back to our respective home cities (luckily enough she lives within a couple hours' drive) she told me that we'll play things by ear and that she's not going to write me off but she needs time to sort this out.
So long story short (as if that's ever going to happen with me...), if you're a poly person looking to add a third/fourth, secondary/primary etc. what ever you want to call it, don't expect that because someone identifies as mono that they are not open to the possibility of something else. I experienced that paradigm shift personally, and am very happy for it. I have seen other people re-evaluate their ideas of what a relationship can be and change their minds about being mono. I'm not saying it always works out well (only time will tell with J and G, only time will tell with me and my new interest, let's call her H) in fact it often doesn't (as with some previous dating experiences that I've had before meeting H) but the point is it's worth a try!
So far as broaching the subject with a possible love interest, it's true that laying it all out from the very beginning will scare some people off. While some people might find this the wrong way to go about things, (and I'm not going to try to say this IS the right way to go about it because I'm not sure myself) I try to give a person a little bit of time to get to know me first; if you think someone is interested in you, or you know they are, let them get to know what kind of person you are at least a little bit before you have 'the talk'. Don't let things get physical and don't let this person get too emotionally invested in you, or you them. I'd say give it a couple of dates/outings, there's no right or wrong number, but don't drag your feet about it under any circumstances. Don't lie; if this someone asks you if you are involved, then it's time to have 'the talk'. If this someone comes right out and tells you how much they like you, that's an appropriate time as well. If you feel like you really like this person and you could see them potentially being part of your life, it's definitely time to tell them. In this situation it is always best to bring it up too early rather than too late because if it's too early the worst that could happen is your potential love interest will simply walk away, if you leave it too late then you're acting unethically and you're going to hurt someone and quite possibly yourself too. It's a fine line to walk because if you bring it up on the first date, the person may like you but they may want to cut and run rather than have to deal with uncomfortable emotions and the intricacies involved in a poly relationship; people stick to what they know and are already comfortable with. If you bring it up too late, the person will feel like you've deceived them (and if the dating/hanging out has turned into a relationship and you still haven't told them, then you ARE deceiving them and it IS too late). Like I said, I'm not sure if this is the right, ethical thing to do, but since much of the normative North American culture is based on a mono philosophy and many, many people have misconceptions as to what it means to be poly, I feel like it's the best workable, ethical, and moral solution. The ideal way to do it would be to meet the person, talk to them for an hour or so to see if you even like them, and then lay it out point blank and let them take the ball from there, but in my experience that simply does not work out. I have gone on dates and laid it out point blank before the evening was over, and that I can say does not work. I have tried the online dating scene and have explicitly wrote in my profile that I am a polyamorist; three guesses how many replies I got on that site.
On the other side of the coin, it wastes a lot less time and emotional butterflies if you are up front about being poly, and any people who are still interested after you tell them are much more likely to be ok with a poly relationship. Again, I have to stress that I am of two minds about the approach that I've laid out; on one hand I stand a higher chance of getting what I want, on the other hand I might piss somebody off or hurt them emotionally. So far I haven't hurt anyone or pissed them off with this approach, mostly just delayed the inevitable, "I don't think I can do that." response and been disappointed, so in that aspect it has been successful for me. On the other hand, I feel like if I had told H at the outset that I was in a relationship at the first sign she might be interested in me, I highly doubt she would be giving us as much thought as she now is and I know she doesn't feel deceived or hurt because I asked her about it.
In any event, my two cents...
R