In terms of the sexuality of the high elves, let me make my case clearly.
I consider them to be very greek in their politics, with relatively few influences from other cultures.
politically the high elves are ruled by the Phoenix King when viewed from the outside. However, I think that his "real" power was far weaker.
Each region of Ulthuan might be called a nation in and of itself, ruled by their respective rulers. (not only does this explain the cultural differences between the various regions, but it also explains the abundance of princes running around). They went to and fro about their various concerns, each keeping much to themselves before the rise of Aenarion, and mildly drifting apart now that a certain stability has been reached with Finubar at the helm.
When you look at it from this perspective, it makes sense that each nation has different specialities, in which they have grown great.
In terms of cultural acceptance of paraphilia (sexual deviations) I would like to make a nation-by-nation assesment of them, starting with Lothern.
Eataine, being the greatest port-state of Ulthuan, followed only closely by Cothique, it is subject to many outside influences and would care greatly for their appearance. Dock-workers and common traders are not particularly interesting but the courts are a magnificent display of power.
In these courts, men and sometimes women vie ferociously for the favour of the Phoenix King, and each would have their own quarters.
Undoubtedly they would also have attendants and some of the less savoury princes or representatives would likely have concubines and a wife. These concubines would be kept hidden amongst the staff, and would rarely be talked about as it would be considered "unfaithful" to openly admit this.
going further north, we come to Caledor, land of the arrogant dragon-princes. These princes would likely care little for what others percieved of them and might smugly pleasure themselves any way they wished. One can only imagine that the arrogance of not lowering their banner for the Phoenix King, would trickle down into their very society, with the poor being downtrotten and nearly mistreated while the rich indulge in various acts of pleasure all year round. Their courts would be much like the courts of Beowulf, once he defeated Grendel.
The populace of the land would seek favour with each other too, by wedding out or offering their daughters to the princes and smalltime landlords.
Then we arive at Tiranoc. A nation falled from power. With the destruction in the sundering they were dealt a grievous blow and their political, financial and economic power destroyed. They would be poor, almost peasantly to the eye of other princes. This has caused a certain faithfulness that is rarely seen amongst the elves. Each house keeps to themselves and they are more concerned with rebuilding their power than earning any favours with each other.
Then we come to Nagarythe. A land utterly devoid of moral or political guidance, and thrown in extreme turmoil. The scars of the dark-elves are still visible there and so too in their population.
Many have taken to forming bands of bandits and are lurking the streets for anyone who might be considered a spy. This distrust would undoubtedly drip down into their relations too. I can imagine that the Nagarythe people are slow to earn the trust of, but once earned they would be an invaluable ally despite their brutish ways.
They would remain extremely faithful to their spouses, and any sign of unfaithfulness would be considered a betrayal and a disgrace, having had the history of the cults of pleasure and morathi right in on their land.
if you travel on, you enter Chrace. As polar opposites of their neighbors the Chracian hunters are strong and stury. While they bore the brunt of much of the early druchii attacks, they remained reclusive and kept to their own forming prides in their courts. A single great hunter would rule over his pride of weaker and lower hunters and wives, emulating their revered White Lions. They are a very careful nation, that is quick to anger, but loyal as a friend. This would also be reflected in their sexual lives, as any unfaithfulness would be considered a challenge to the alpha and they would battle to find out who is stronger.
The influence of the phoenix king would be hardly known out here, and so too the standards of Eataine.
Enter Cothique. Like Chrace their land is beset by conflict with the Dark Elves, but the Cothicians are hardly folk. Their ships are filled with men riding the waves of the sea with a smile on their face as rain batters down on them. Like no other they know the perils of the sea, and they consider it a worthy death to be buried at sea.
All things considered, homosexuality would not be uncommon in this place, as often for long periods of time, the men would be separated from their wives on ships, and would be gone for months many even for years or decades. They would come to trust and love each other as men, and this separation, while innitially driven by loneliness and lust, would turn into a culturally acceptable behavior after millenia of understanding.
Their military structure too would be similar, being drafted into a regiment would mean separation from their wives, as they are to reside in the baracks, while their wives and children would tend to the land and home. It would not be uncommon to find yourself in a regiment with your cousin to your left, and your father to your right and a brother to your back.
While bordering Eataine again, Yvresse is devided on it's influence of the Phoenix Court. To the north, their lands would be greatly similar to Cothique, and inter-sexual and inter-mariage sexual relationships would be more common, while to the south it would display the nearly prudish ways of the Phoenix Court, seeking favour with their high king.
Across the annulii mountains, as seen from Yvresse lies Saphery, and land steeped in magical knowledge and where scholars and mages are most prolific. They are known for their craft in the magical arts and is the home of the famous Tower of Hoeth.
The Phoenix King's unfluence would be great here, and the mages would be as carefull about their sexual deviance as their western neighbors, keeping a good front and practicing care with their relationships.
Avelorn, the great forest and home of the Everqueen. The maiden guard patroll the roads and keep a watchful eye, while the men do a large portion of the "common" works compared to the other nations. They would be an almost equalised nation, where men and women are free to choose with whom and what they do it, taking the example of the Everqueen and her political relations with the Phoenix King and her personal champion Tyrion.
Ellyrion, home of the famed ellyrian reavers, and the greatest horsemen around. One can only wonder if their would spend the same amount of time with their wives as they do with their horses. I think the Ellyrians would be openly loyal to their wives, and some might have a sexual relationship even with their horses. While this is far from the norm, it is probably more accepted here than in any of the other nations, save perhaps Tiranoc.
With that, I conclude my speculations on the sexual nature of each nation and the influences they have on each other. Feel free to disagree, but I hope this explains some of the stances I take when writing my stories.