Well if they can have casual sexual relationships but choose not to, I personally don't see why you can't choose to have casual sexual relationships. Your partners may have conscious or subconscious expectations that as they have not gone outside your relationship you won't either. Or they may have fears of being replaced or having even less time with you. Try to have some frank talk about fears and concerns.
Also, do you know why you crave flirting and light sexual contact (kissing etc.)? Some people are more flirty than others - they like getting and receiving that kind of attention. And there is nothing wrong with that. Perhaps you could satisfy that want through flirting but not actual sex? Also maybe you are not getting enough touch in general? If you are someone high in the physical touch love language even with 2 partners you may not be receiving or giving enough touch to satisfy you. I find if I am low on touch, I crave sex intensely. If I am more full on touch, I still want sex but it is not a blinding WANT NOW. Having my touch needs more in balance makes my lust way more manageable.