Hmmm...duogamous implies only two and though this may be more accurate for some, I find that those who stray don't just stray to one - perhaps one at a time but not one only. Maybe multogamous.
According to Thesaurus.com, multi suggests:
Definition: complex
Synonyms: advanced, complicated, delicate, elaborate, highly-developed, intricate, involved, knotty, labyrinthine, modern, multi-faceted, refined, subtle
Antonyms: naive, unsophisticated

I kind of like how "multi" suggests a kind of snootiness.