Is it gross for a 20 year old to date a 17 year old?

My little brother is 17 and lives on his own. He turns 18 in May. I went to his house and used his spare key because he wasn't answering the door. Walked into his room and caught him shagging some bitch who clearly wasn't a high schooler. I walked out and waited for them to change. He came out and caught an attitude with me. I asked him how old is that girl and he said 20. Then I shoved the woman harshly because why tf are you bouncing on a minor's dick? She shoved me back so I shoved her ass harder this time and my brother picked me up and took me to another room because he knew I was gonna whoop that girl. I told him that she should be prosecuted and he told me that their relationship is legal. LEGALITY DOES NOT MEAN MORALITY. When I was his age do you guys know how many 20+ year olds I fucked because I thought I was grown?? Now I'm 24 with a baby and I realized how fucked up that situation was. I was a damn child and should've been acting as such. My brother thinks he knows it all and it breaks my heart but I guess I can't do shit besides be his shoulder to cry on wheh this relationship fails badly.