The timeline is so screwed up when it comes to the "details" of Sebastian's "past".
See, the way Sebastian talks about it, you'd think he had been a debauched hedonist/lady killer for a long time.
Except that doesn't fit when you add the bit about his grandfather's bow. He was thirteen when his grandpa promised him the bow and said grandpa supposedly died before Sebastian could show him his improved skills. Not only that, but Sebastian was sent to the Chantry at around the same time (if not before).
Meaning there is a very short window for Sebastian to be a wild child problematic prince, actually.
It is highly improbable that a 13-year-old is getting up to the kinds of sexual hedonism Sebastian implies, unless that 13-year-old is being sexually abused. And I don't think the writers intended Sebastian as an SA victim (mainly because it naturally follows that his parents are VILE victim-blaming pieces of refuse).
So he needs a minimum of two years before getting sent to the Chantry. That minimum leaves him one year of potential sexual hedonism. True, 13-to-14 years old do "experiment" but again, what Sebastian's implied sexual past is far, far beyond tween-to early teen "experimentation". (And again, I don't think Sebastian is an SA victim).
However, Sebastian can't have been older than 16-17 when he got sent to the Chantry. Because while I'm no archery expert, an interval of 4+ years is too much time, in my opinion, for Sebastian to show off his skills to Gramps.
This is not to say Sebastian was a virgin when he entered the Chantry. But I don't think he is nearly as sexually experienced as he leads us to think. In fact, I believe the best explanation is that the "sordidness" is highly exaggerated due to Starkhaven being a more sexually conservative environment.
And why do I think Starkhaven is especially conservative? Think about it-his parents' entire motive for sending Sebastian away was that they believed he "might" produce a bastard. Not because he did, but because he might.
Meanwhile, let's take a look at Ferelden, which is relatively sexually conservative compared to Orlais (especially if Zevran's comments are any indication).
They have zero problems accepting Alistair's rise to the throne despite him being a bastard. Granted, Ferelden is in a massive crisis when this happens, but still. Nobody bothers to try and comb through the Theirin genealogical records to see if Cailan has any cousins. True, Ferelden is in a bit of a crisis, and Cailan probably has no cousins.
But Starkhaven was also in a crisis, and searching for a cousin was exactly what they did. Even though Sebastian (who, for all we know, is perfectly legitimate) is still alive.
And even after Lady Harriman is disposed of, said cousin is still on the throne. (What happens to the poor guy is never explained). Nobody bothered to try and even drag Sebastian back to Starkhaven, never mind give the throne to a potential bastard sibling of his.
Point is, while Ferelden appears to frown on extramarital sex, it will still consider royal bastards to be perfectly acceptable emergency heirs. Whereas in Starkhaven, the mere possibility of royal bastards will get the offender kicked out of the family altogether.
The latter sounds pretty sexual conservative in my view.