It isn't Unforgivable in the way that the Unforgivables are
A popular fan theory is that there is no grey area with Unforgivable curses. They do what they say on the tin and nothing else. Killing curse kills, imperio controls, crucio tortures. The caster also has to fully understand and mean to cause the effect the curse entails too. There's no excuse for fully casting them other than they wanted to kill/control/torture.
This is alluded to in OotP when <PERSON> taunts <PERSON> about his weak/failed <PERSON>:
Hatred rose in <PERSON> such as he had never known before: he flung himself out from behind the fountain and bellowed, "<PERSON>!"
<PERSON> screamed: the spell had knocked her off her feet, but she did not writhe and shriek with pain as <PERSON> had – she was already back on her feet, breathless, no longer laughing. [...]
"Never used an Unforgivable Curse before, have you, boy?" she yelled. 773
She had abandoned her baby voice now. "You need to mean them, <PERSON>! You need to really want to cause pain – to enjoy it – righteous anger won't hurt me for long – I'll show you how it is done, shall I? I'll give you a lesson —" (36.30-32) Order of the Phoenix
And again with fake!Moody in the Goblet of Fire where he claims during the Unforgivables class that if the class were to all point their wands at him and say "<PERSON>" he wouldn't get much more than a bloody nose - they'd have to mean it for him to die (unable to find the quote for this, sorry).
Sectumsempra is different:
<PERSON> is able to fully cast Sectumsempra off-hand and in a panic without understanding any of it's effects. Plus the curse is ambiguous as to how much damage it does, and as <PERSON><PHONE_NUMBER>) Order of the Phoenix
And again with fake!Moody in the Goblet of Fire where he claims during the Unforgivables class that if the class were to all point their wands at him and say "Avada Kedavra" he wouldn't get much more than a bloody nose - they'd have to mean it for him to die (unable to find the quote for this, sorry).
Sectumsempra is different:
Harry is able to fully cast Sectumsempra off-hand and in a panic without understanding any of it's effects. Plus the curse is ambiguous as to how much damage it does, and as Snape proves the damage is reversible. This makes it on par with spells like Reducto, Confringo, or Bombarda Maxima. Each of these spells could be used to decimate another wizard, but also could be used to wound instead.
I would imagine in the HP universe there are more than just three curses that you aren't allowed to use too - the Unforgivables are just the worst and so are on another level. Sectumsempra would most likely regarded as Dark/illegal/restricted, along with other spells such as the organ-expelling/skin-boiling curse etc. 773