Because the people on Game of Thrones are not real and are actually played by a special subset of superior humanity called "actors" and "actresses," the impression the show may have given you of Septa Unella is not actually reflected in her daily life.
Unlike the dour Septa Unella, all actresses are gorgeous, with pretty much no exceptions. That's a great example of sexism, since John C. Reilly is a famous actor and he looks like a mountain goat. But the point here is that Septa Unella is a beautiful woman when going by her human name, Hannah Waddingham.
She's not constantly following Queen Cersei around ringing a bell to call the peasants' attention to her sins, and she's not always wearing a baggy habit.
As the Daily Caller writes in words that are much more frank and weird than necessary: "The Actress Who Plays Septa Unella in 'Game of Thrones' Is Actually A GORGEOUS SMOKESHOW." Nice, real classy. (They should have gone with Lord of Bones, right?)
Game of Thrones is notorious for gratuitous nudity, and Jamie and Cersei already had sex in a sept, so will the extra-long season finale mean nudity for Septa Unella?
Because this is already getting really creepy, how about you all agree to just watch the next three episodes and find out for yourselves.