Well that's the monogamist mindset. We all fantasize about fucking attractive people we see, while staying blissfully in love/lust with our faithful SOs.
Successful poly people, OTOH, get lots of vicarious pleasure (compersion) out of their partners' other romantic/sexual relations.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags, F, 60, poly-dating, loving and living with
miss pixi, F, 38
Punk, 41, M (dating since Oct 2015)
and a few more casual relationships