There was a post I read in a forum that was vociferous in its defense of mono/poly relationships. Seriously cursing, no punctuation, no caps, no style, sort of argument. It was truly appalling (the poster, not the idea). The basic point of contention was that they had gotten static from other poly people telling them that they were doing poly "wrong", that they were being selfish. Since I've heard this from other poly people, I guess they may have had a point.
We all have the right to do our relationships anyway we see fit. However, the assumption, even among the poly community, is often that in a mono/poly pairing, the mono partner is sacrificing something, perhaps by being in a relationship style that doesn't suit them personally, to stay with the poly partner.
Some of the more functional poly folk I've met are with mono partners. The way this seems to work best is if the mono partner doesn't perceive sexuality as really that much different than any other form of socialization. Emotional connection is seen as an abundant commodity, and their poly partners do an exceptional job of creating a base of trust to work from. They need to be willing to speak up for themselves and have well-defined boundaries, including the capacity to negotiate and form connections with their metamours. In short, all the qualities of any well-functioning poly partner, but choosing to be monogamous themselves.
That the mono partner might change their mind and exercise the option to become poly is also something that seems important to mono/poly pairings. Where knickers get all twisted up is when the idea of choice is taken off the table. Even if it's not exercised, it needs to be there.
Being functionally poly in all directions myself, I don't have a lot of experience to draw off of when it comes to mono/poly. What direct experience I do have comes from spending time where one of my partners wanted to manifest an outside relationship and wasn't able to at that time. Either there were too many other responsibilities to afford the time to connect with someone, or there wasn't a suitable candidate to bring on board. It seems we're all agreed that it's better to be "poly single" than to insert a poorly matched partner, or try to satisfy too many demands on one's attention.
When you are the poly partner in a relationship where your partner is, at least in practical application, mono, guilty feelings may arise about having something in life that one's partner also wants. Another possibility is being in a position where one is looking forward to their partner having a secondary, not only for the positive impact on their growth in life, but also to alleviate pressure on the relationship from being the footloose poly partner while the mono partner is all nose-to-the-grind stone. The amount of concern that a poly partner might feel, the desire to make sure the mono partner is "really okay", can branch into martyrdom or taking on responsibility for the feelings of another.
As the mono partner, it is imperative to develop coping strategies and a strong support network outside of your poly partner to help work through the challenges that are primarily your responsibility. While there are a great many things that are within the purview of the relationship, there are pieces that belong to you, and would be best served with outside input, or structured action that helps you work through your own emotions.
Let's run an example: Let's say your partner is off on a date. You've asked for what you needed as far as support from them, and are still having feelings of anxiety. What do you do? Some strategies that may be helpful include setting up a different social opportunity for yourself, gaming, catching a movie, working on a project, cleaning, journaling, or working. If it's something that you can feel good about, plug it into that space and give yourself some emotional breathing room. Again, these are the same sorts of strategies that poly/poly partners would want to develop, so it can be very useful for mono partners to participate in poly groups, gaining these types of skills that serve their personal needs.