I'm pretty sure Frieren acts like nothing happened because she's generally nonchalant about things at best and socially an old lady at best. A lot of her sensibilities are just different compared to a human specifically because she lives so long, which is one of the main plot points for her.
Someone in a discord I'm in said it best, basically saying that since she's an elf she kind of views a lot of things humans find embarrassing with a bit of detachment or in a "that's dumb" kind of way. Kind of like how a parent would see a kid do something and think "aw isn't that silly or cute."
She can obviously understand human sexuality, since she tried to give Fern the potion that removes clothes so Fern can give Stark a "Gift" for his birthday. But that's a version of Frieren who saw one of her best friends die and has made an effort to learn more about humans overall.
Frieren during her journey was a shut in elf that only started interacting with people outside of business transactions or necessity because she was recruited to fight the demon king. After her master died she literally wanders around and does her own thing for hundreds of years, the 10 years she spends with her party were probably the most social interaction she's had again, since her master died.
So it makes sense that she doesn't really have a problem with it. Also Frierens outfit is closer to a robe or a dress than a skirt.