Why is my sister mad?

My sister's boyfriend, Ray came over for the weekend but he forgot to bring his sleeping shorts so he put on my sister shorts, but the shorts were very loose.

Me and my sister share a room so i was going in and out, getting ready to leave and Ray bent down to grab his pants off the floor so i got a glance of "Dong" poking out through the leg holes.

I was a bit shocked and disturbed so i told my sister later that he shouldn't wear those shorts any more, i was kind of traumatized and it made me uncomfortable and she got mad at me.

She told me that " don't be traumatized it's not like that's the first time you've seen a penis. okay he can wear whatever he wants" in a harsh voice.

I don't know why she's mad at me...and also at night they have sex right next to me since we share the same room. this is so unfair....