Well, they'd said before this that Tevinter was harsh on homosexuality among nobles. You can do whatever you want with the slaves, but nobles are expected to marry and breed more mage babies, basically. Their caste system is based on who can breed the most and best mages through family lines, so it's a big deal to them. It's similar to the situation with Qunari breeding programs. 
Yeah. I'll quote World of Thedas:
Typically, one's sexual habits are considered natural and
separate from matters of procreation, and only among the nobility,
where procreation involves issues of inheritance and the
union of powerful families, is it considered of vital importance.
Yet, even there, a noble who has done their duty to the family
might be allowed to pursue their own sexual interests without
raising eyebrows.
The view on indulging lusts with a member of the same gender
varies from land to land. In Orlais, it is considered a quirk of
character and nothing more. In Ferelden, it is a matter of scandal if
done indiscreetly but otherwise nothing noteworthy. In Tevinter,
it is considered selfish and deviant behavior among nobles, but
actively encouraged with favored slaves. Nowhere is it forbidden,
and sex of any kind is only considered worthy of judgment when
taken to awful excess or performed in the public eye."
One might assume Tevinter nobility would be even more obsessed with continuing the family line than other noble houses in Thedas, considering that magic seems to be an inherited ability and (if Dorian is any indication) its potency might grow with assiduous political marriages.
(I could almost imagine nobles trading their children like people breed horses - trying to get the best match to ensure the most powerful children.)