It has been said by some, and by me once or twice, that the worst thing about polyamory is monogamy. Falling for monogamous people who won't shift is really hard, and monogamous people who try out poly and then don't deal well with it, can break your heart when they leave.
I've done both of these. I've fallen for a monogamous man and I've been left by a poly partner for a monogamous partner because she couldn't cope with my partner's polyamory any more... and he didn't want to lose her, but was happy to lose me (I could rant about this, but I won't other than its short-sighted and blind behaviour - denying an aspect of yourself doesn't lead to long term happiness).
Anyway, my point is this... time leads you down all sorts of strange roads, and that person who can't consider polyamory today or can't cope with it today may actually grow and think and learn about it and decide, in time, that they can.
This, I think, is the best way to go about it. Forcing someone to be poly isn't going to work for them or for you. It's a big change to stop following the footsteps of society and look at relationships in a different light. It's a big step to actually ask yourself if you could share a partner with someone else or even be shared by other people. For some this comes naturally, for others it's weird, odd and very scary.
People's choices as well as your own integrity should be respected in this instance. You should be who you are.
If you are poly and in relationships with other people, then leaving those people for a monogamous relationship isn't necessarily good for you, the other people or the person you are hooking up with . The big picture is important, I feel.
If you are monogamous, then you should respect that about yourself... and if you are questioning it, talk freely to the person/s you are interested in and go from there.
Just like sexuality, I think that polyamoury is a fluid scale from people who are very poly, who would never ever be happy being monogamous to people who are so monogamous that the idea of having an affair or another relationship is repulsive to them. Generally though, most of us fall into the spectrum. From the people who understand it but don't want to do it, through to those who do it and are hoping for the best, and then those who are poly and actively making it work and are happy doing so.
So, back to my point. Time. There is time enough for everything. Don't rush things, don't feel that you must make a decision today, tomorrow or even next year. If someone you love is poly and you are not... then question what will make you happy and whether you can live that lifestyle:
- being monogamous with them while they are poly with others, and don't forget that appropriate boundaries still need to be set;
- learning to be poly yourself, and all that entails;
- or if you can't do it at all.
If you are poly and have fallen in love with someone who is monogamous, then respect their beliefs and values, even if you don't hold them. Tell them about what poly does for you, but let them decide, when and if they are ever ready, to come and join you or not.
One of the great things about being poly is that you don't have to fuck everyone you love. You don't have to have any more than a deep, loving and committed friendship. Sure sex is great, but it isn't the sole reason you have relationships with people. It may be the reason you have relationships with some people, but not all of them.
The most important thing you can do for yourself and the people you love, is to aim to always do the right thing by them and by you. Keep your integrity and only question your own values if you really want to and think that perhaps it might be time. Don't let anyone push you into something you don't want to do. You are the one who best makes you happy.
That said though, finding out what you want is an important part of the journey of life. Experimentation is not a bad way to go about these things, but try to do it respectfully of other people's feelings. Many poly people I know, including myself, fall in love hard and deep. Communicate clearly what you are doing so that everyone involved knows what is going on and can guard (or not) their hearts as much as they need to.