Is it normal people treat me as the most stupid person on this earth..

When I was a kid I had to move out of countries a few time. When my family finally settled down two years of my schooling were wasted because here in the U.S they wouldn't accept it. They said the studies were different and even though math’s universal apparently it’s different to! I was held back two years at a young age now after five years I'm in high school. I'm a junior and I'm 18. Sometimes people treat me like an idiot because of that or at least it feels as if that’s the case. Not only that I was born in a family where I wasn’t really told anything, never had the sex talk or anything. I didn’t know a thing about anything pertaining to sexual activity. When I was in 9th grade I learned what the word intercourse meant. Then when people would find out they'd start asking me if I knew words such as “blowjob” or “masturbation”...It was really awkward when I said no they would start explaining as if I'm a retard using slow pronunciation... I felt like an idiot. I still feel uncomfortable that people undermine me because I don’t like to say words like “penis” or “vagina.” Now that I'm 18 and a few people who know sometimes they put me on blast when they are with other friends. Sometimes this scenario happens: (P1) says "she’s 18 and she doesn’t even know this is…" and I can see the p2’s surprised face while p1 says "I know right" and p2 says "I hope I'm not 18 and a junior" or" I hope I don’t end up like you" I feel as if people treat me as if I'm a moron because I'm 18 in junior year and I don’t have much knowledge of what I consider "dirty". Sometimes my friends have weird conversations where they will have role-playing moments and pretend they’re doctor-patient and they start joking around with something really perverted.. when I don’t get it sometimes they’ll explain and when I’m grossed out they’ll look at me as if I’m being really immature and stupid.. I asked why do they treat me as like this and then this guy said people would treat me more seriously if I weren’t so illiterate in the sex section. I feel really insecure about my age now and I hate it when people ask me something and laugh at my face for not knowing it... I feel as if I’m treated as the most stupid person on earth… is it normal? What should I do please help... am I really immature for not knowing these types of stuff?