There was a scene in an episode of Star Trek: Deep Space Nine (the sexiest Trek, IMHO), during the time when Worf has begun to court Jadzia. They have a date, and he goes to her quarters to pick her up, and when the door opens, a muscly bald guy leaves. He and Jadzia are all sweaty and she is toweling herself, and says "thanks for a great workout" to the guy who's leaving, but it was something like wrestling they were doing and the way it all came across was very sexual. She's totally unselfconscious and unapologetic and doesn't understand why Worf is jealous. I think there were a few more instances where Jadzia's "friendships" with other males come into question in that episode.
There was a species with a polyamorous culture on Star Trek: Enterprise. The Denobulan tradition is that every male has three wives, each of which has three husbands. Dr. Phlox on the Enterprise is Denobulan. I just happened to catch an episode with some delightful scenes with poly in its storyline. Dr. Phlox's wife Feezal is visiting the Enterprise while it is docked somewhere. I transcribed two fun scenes:
DR. PHLOX is in the sick bay. COMMANDER TUCKER, the chief engineer, stands at the entrance and asks if he can speak to Phlox privately. Phlox welcomes him in. Tucker enters.
TUCKER
Malcolm says this is a bad idea, but
I think it's the right thing to do.
PHLOX
What's that?
TUCKER
Feezal... I mean, Mrs. Phlox... is a
lovely woman and she's very smart.
She knows more about quantum
optics than anyone I've ever met.
PHLOX
She is remarkable.
TUCKER
Remarkable. Yeah, she sure is.
TUCKER appears bashful.
PHLOX
Was there something else?
TUCKER
(visibly uncomfortable)
There is. You've got to understand,
I've been the perfect gentleman.
Absolutely nothing's happened.
She's trying to... she's... she's um,
making advances, if you know what
I mean.
PHLOX
Sexual advances?
TUCKER
I'm afraid so.
PHLOX
(grins widely)
Has she offered to give you a rose
petal bath?
TUCKER
No, no, nothing like that.
PHLOX
Oh, any man would be a fool to
ignore the romantic overtures of a
healthy Denobulan woman. Haha!
Tucker hangs his head.
PHLOX
(perplexed)
Don't you find her attractive?
TUCKER
Sure! I mean, no, she's your wife!
PHLOX
(shrugs)
What does that have to do with it?
TUCKER
She's your wife?
PHLOX
Oh nonsense! Nonsense! You're
too concerned about human
morality. I thought you wanted to
learn about new cultures. Isn't that
why you joined Starfleet?
TUCKER
Of course it is, but I was brought
up believing you don't play around
with another man's wife - and I
don't think I'm ever gonna change
my mind about that.
PHLOX
Your loss.
Phlox shrugs and walks away.
-----------------
SICK BAY - LATER
Phlox and Tucker stand together, looking at a microscope, and make adjustments to some of its dials. The door opens. FEEZAL, Dr. Phlox's wife, enters. Phlox's eyes light up.
PHLOX
(to Feezal)
My beloved.
Feezal and Phlox bring their faces close together and inhale each other deeply (a Denobulan kiss). Tucker squirms, looks away.
PHLOX
I certainly hope it's not another
four years until I see you again.
FEEZAL
So do I. Now, remember, your other
wives are anxious to see you, too.
PHLOX
Commander Tucker assures me
he'll keep your beautiful microscope
in perfect running order.
FEEZAL
As his doctor, I hope you'll keep
Commander Tucker in perfect
running order.
(looks at TUCKER
flirtatiously)
Perhaps that would motivate me
to visit more often.
Tucker looks like a trapped animal that wants to run away.
PHLOX
(to Tucker)
It's a shame you two didn't... get
to know each other better.
TUCKER
(to Phlox)
I've got to get back to my warp
engine. The plasma's running a
little hot.
Feezal takes a step closer to Tucker, looks up at him.
FEEZAL
I know how it feels.
She smiles at Tucker coquettishly. Phlox, on Tucker's other side, grins at his wife proudly.
TUCKER
(to Feezal)
Uh... pleasure meeting you.
He quickly exits sick bay. Feezal stands next to Phlox. They watch Tucker leave.
PHLOX
Humans!
They both giggle.
--------------------
Of course, there's also Londo Mollari from Babylon 5, who was def poly! From Wikipedia:
"His relationship with women is complex, having to manage his three wives and a mistress (Adira). When Londo is granted the favor of a divorce from two of his wives as a gift from the Emperor, two of the wives, Daggair and Mariel, battle for supremacy in the group, each trying to convince Londo that she loves him best so that she won't lose the status the marriage gives her. The third wife, Timov, refuses to play along, making her disdain for Londo well-known. However, when Londo is poisoned, ostensibly by Mariel, it is Timov who secretly saves his life by providing him with a blood transfusion. In spite of not knowing this, Londo chooses to remain married to Timov: "Because, with you, my dear, I will always know exactly where I stand." This, along with his relationship with the beautiful Adira, make for a man that accepts the truth over beautiful lies and who respects the women he loves and who love him truly."