I actually don't think إخلاص is appropriate for either meaning. Monogamy means that you have a single partner; it's not about fidelity. If you are in a monogamous relationship and you cheat on your partner, you do not all of a sudden become polygamous. Thus, the translation should only refer to the fact of having a single partner, with no reference to fidelity.
Wikipedia has زواج أحادي for the “marriage” meaning.
Thank you for reporting. The rendering زواج أحادي has been added to the dictionary. An additional suggested rendering is ارتباط بشريك واحد. The renderings will be visible online after our next update, within a few months.