Our mindset is poly, but we've had long stretches where it was just the two of us - monogamous in practice but not in principle. When we first met, she had a long term boyfriend, and I soon met a long-term secondary woman. That latter lasted for several years, but her bf found a primary early on and moved on. Lately, I've again had a secondary for two years, but my wife has had little interest in finding anyone for herself. We are older, so perhaps it's hormonal influences for her at this point.
I think that having or wanting someone else can ebb and flow over time, and it seems that both don't always have someone at the same time. If it bothers you to not have someone else while your partner does, then it may cause some problems - especially if they are not sensitive to the situation. Talking it through, and mutual reassurance helps a lot, as does recognizing that these cycles come and go for many.