I agree that teenagers should be taught all the options, realities, and responsibilities as well. I learned early on that love wasn't enough, but was also taught that once you have children with someone, you've committed to staying with them through the children's entire childhood, in spite of abuse because it is better for your children to have both parents than only one, no matter how bad the relationship between the parents is. I personally never prescribed to that belief, but it is what was modeled for me.
Runic Wolf figured out that I was bi-sexual before I did and our initial talks about opening up our marriage were because he felt that I deserved the chance to be true to myself. (I love him for that.) For me, being polyamarous means that I have the ability and desire to love multiple people at the same time, but living polyamarously means that I have the desire to have multiple loving relationships. The same is true for Runic Wolf.
I will have to disagree with you on your statement that "implicit in monogamy is the understanding that you both will have feelings and desires for other people but are choosing to commit whatever amount of resources you choose to each other. Neither romantic feelings nor sexual desires for other people are in any way threats to monogamy." Not on the part about committment, but on the understanding that you will both have feelings and desires for other people. Most people are raised with the belief that when you find the "ONE" you will not have any feelings or desires for anyone else. So they are hurt when their partner expresses that they do have those biologically natural attractions. Some couples handle this better than others. But I would say that most monogamous people do feel that romantic and sexual desires for other people are a threat to monogamy and specifically to their relationship. Whether or not that is true, that is their perception.