If I'm up front at the start (and all subsequent stages) of a relationship about the fact I don't value/require physical monogamy, about the fact I like to meet and flirt with new people, about the fact I like physical intimacy (not necessarily ing, but also hugging and snuggling) even in my platonic friendships, and about the fact all of this can/may lead to my wanting to to sleep with someone else, then someone deciding to enter into a relationship with me even in light of this information should know full well what they're getting into. If/when they then decide to end the relationship because a situation I've prepared them for actually happens, it's absolutely not the same thing as me leaving someone to go someone else.
Basically, yes.
Absolute physical/sexual monogamy doesn't work for a lot of people. I've recognized over time that it doesn't work, and even more importantly, isn't necessary for me. Disclosing that information and defining a relationship structure that works for both people in a relationship, and sticking to it, is in my opinion absolutely the mature and moral course of action.