Different poly couples get different things out of having another partner or partners.
Since my gf and I are both bi, we both crave men for obvious reasons. Also, I was in a 30 year marriage before I met her, and now that I dumped him, I wanted to play the field, experience attraction, sex, have some laughs and maybe someday some true love with a suitable guy.
My gf is male to female transgender and has only started living fulltime as a woman for 2-3 years. She's always been poly and dated both genders (and a few in between) when she was androgynous, but now she's loving having a bf because it's gender affirming. Also she loves the taste of hetero-privilege she gets when they go out in public. She's tiny, 5'4", and he's massive, 6'5" and burly. No one gets to mess with her or question her gender without going past her bf!
(When we go out we are usually mistaken for mother and daughter because of our age gap. I'm 55 and she's 33. Her bf is only 35, so they make a cute MF couple.)
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.
~Percy Byshe Shelley