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.