In Dragon Age: Inquisition, you learn more about modern Tevinter nobility because of Dorian, a mage who joins your party well away from his homeland. You discover that he has many reasons for steering clear of his family, and his sexual orientation is one of them. For nobles in particular, homosexuality is seen as a terrible flaw. They are meant to birth the next generation of the ruling class, but more than that, they are supposed to pass on the very best genes, including any gifts for magic the bloodline may possess. Their marriages also cement alliances between families with the most to lose.
Anything that gets in the way of procreation in their ranks is severely shunned, which means that the more a noble does not wish to perform with the opposite sex within a sanctioned marriage, the worse they are regarded and treated. As Brother Genitivi reveals in his treatise on sex, Tevinter nobles are actively encouraged to express their sexuality with slaves, including homosexual desires. So long as they live the rest of their lives as their station demands, they can have whatever flavor of private fun they wish with the people they own. For some slaves, sexual activity is one of their primary functions.
For some nobles, living in such a way is just not acceptable. They rebel, refuse, or at least delay getting married. They pursue lovers they are forbidden to approach. Some are simply unable to perform sexually with someone they are entirely unattracted to. Inquisition reveals that a Tevinter noble parent is not above using blood magic to try to change their child's sexual orientation, as shown in the screencapture above. In the modern world of Thedas, many Tevinters are at least wary of blood magic for how it can go awry. The game shows us, however, that changing such an ingrained part of a person is not possible, even with blood magic. This is worth noting.
But what does this mean for the ancient Tevinter setting in which my game is running, before the First Blight? How was the same situation different in the bygone Imperium? I would say that, if anything, nobles were not yet so obsessed with trying to breed only the very "best." They knew they were the best and had ruled long enough to prove it. While the right match was important, they didn't have quite as much to prove, and weren't as far from the ancient glory days to be so desperate. They tried to stick to the "right" families but could experiment a little more, especially the further from Minrathous they were. And as long as they bred, they had less to lose by being gay.
In 800 TE, Tevinter nobles were fresh out of a civil war that nearly tore the Imperium apart. Marriages were key to reconciliation, but resentments ran high between families that had been feuding viciously, so many unions were tenuous at best. Lovers of all descriptions could be used to insult spouses and once children were born, noble parents could withdraw back to their own interests. In this particular era, however, with a number of nobles perishing in sabotage of the civil war, breeding was encouraged with praise, perks, and advancements, especially if magical potential resulted.
In short, as long as a noble adult was fertile and a parent, their sexual proclivities were their own business. If they were young and infertile, they were expected to be searching for a cure. If they had not produced children yet, they were pressured to do so. If their children perished and they could still breed, they were expected to produce again. Dallying in barren unions at a childless stage in life resulted in penalties not unlike the ones Dorian faced many centuries later. An heir was a noble's ticket to a greater modicum of freedom, with family members and slaves to pick up child-rearing duties - at least until offspring proved interesting or troublesome enough to get involved with.
And to lighten the tone a bit, here's something in this vein that's fun: