The pashtus is that the din that a married woman must cover her hair is not because of erva, and it is specific to a married woman outside of her home (learned out from Sotah).
There is a separate halacha that hair is erva, meaning that it can provoke hirhur and saying kriya shema in front of it would be a problem.
Why then are people not makpid to avoid davening in front of single girls’ hair? According to the Aruch HaShulchan and R’Moshe Feinstein, when the Gemara in Berachos says “the hair of a woman is erva” that means that “the hair of a woman could be erva.” Meaning, the Gemara had just said tefach b’isha (in areas normally covered) is erva. I might have thought that this only applied to actual skin, k’mashma lan that it applies to hair (and singing) as well.
But this halacha of erva only applies to areas that are normally covered. So in a society where single girls do not cover their hair, their hair will not especially cause hirhur and will not be erva. But married women are required to cover their hair whether or not it has the status of erva.