It wasn't male-male intercourse in and of itself that upset the Romans so much as any kind of gender non-conformity. The idea that a man (a real man, not a eunuch) would accept penetration by another man or being put in the womanly position during sex was deeply unsettling, and occasionally punishable by death.
As said above, that is a very specific situation. The point originally made was that the guy in question, as a free and noble scion, would have been free to have intercourse with the person he preferred, as long as he also provided an heir.
"Decadence" is a 19th century (or late 18th century at best, although its connotations of sexual permissiveness are very 19th century) concept. Applying the concept of decadence to any pre-19th century society as an argument as to why a fictional society based on it should be sexually permissive is an anachronism. Romans were not particularly sexually permissive, they just had a different view of sexual morality to the one which would be recognizable to modern Christians and post-Christians.
Well, that's a bit of a stretch, early Christians portraied the Patricians as decadent and dissolute, that is partly why the stereotypes of the orgiastic Ancient Romans exist in the first place. In fact, without even going there, didn't Augustus himself pontificate against the dissolution of his own generation and made laws encouraging regular marriages? Saying that "decadence" is a modern concept is somewhat curious when pretty much every Roman orator went on and on and on about how the Romans of his time had become degenerates and effeminate fools who did not care about the oh-so-precious traditions of their forefathers.
My main point was not that though.
My main point in that example was that the fictional culture moved more or less in line with the stereotypical representation of its real life corrispective culture, but for that single issue. Now, obviously, in real-life Byzantium male on male intercourse was not looked well upon under any circumstance, but real-life Byzantium had had centuries to develop that new stance due to the influence of Christianity, Ancient Tevinter did not.
The excuses that are brought forth in the game to justify their stance on that issue feel weak and far-fetched, which makes the issue stand out, especially since it is given such space and insight during the plot.
It's not that it wouldn't be conceivable that that specific stance was taken by the fictional government, it is, after all, fictional, but the way it is presented does not look realistic to me at all.
Andrastian Tevinter had been depicted up to that point as a deceptive and morally bankrupt society (this both from people that were against it, people that were part of it and codex entries), where everything was fine behind closed doors (blood magic, assassination, daemon summoning, etc.), so, when you want to write in that
one thing where they
do draw their genuine moral stand, I would expect it to have legitimate reasons, especially if it is part of a major plot point.