When they say that a narcissist's ego was stunted at age 3 or 5, they mean every part of them.  For most of us, our mothers are so emotionally immature, that they have no idea how to address anything in this arena.  Or, if they do, they will tell us one thing, and then show us something completely different with their actions and reactions.  Let me give you a few examples of how my mother, so lovingly, addressed my sexual education:

• Masturbation is evil.  And women who do that will cause themselves not to have babies.  This second sentence is exactly what my mother said to me as a small child.  The first sentence is the message that was relayed.  She even backed up her reason by telling me that's why they adopted me, because she lost her first baby because of that reason.  That's probably one of the most fucked up things I've ever heard.  If a) she though that was true?  Then fuck her mother for telling her that!  How horrible to think your own child died because of something our bodies were made to do??  b) she said it as a way to cement her point but knew it wasn't true?  Then fuck her!  What kind of monster lies about something so horrible as to why you lost a child??  Jesus.  Either way, it's fucked up.  She also physically abused me if she caught me doing anything and threatened physical abuse if "I ever catch you doing that."  This happened when I was really little, like 4. 
• Any interest in sex at a young age leads to perversion.  When I was young, I didn't realize my dad was a porn addict.  He had oodles and oodles of porn in his closet: movies, magazines, and books.  So, when I was 11 or 12, I found one of his books and wanted to know what it was about.  Whoah!  That was gross!  I brought it to my friends and we all read it and thought it was hilarious and gross and strange and kind of interesting.  So, when I put it back, I just chucked it into the closet.  I didn't put it back where it went, so my mom found out and took me aside.  "What is wrong with you?  There's something wrong with you, just like your father.  Do you want to be like him?  That's disgusting!  There's something really wrong with you if you like that sort of thing!"  I loved my dad and I didn't mind being like him.  That wasn't an insult to me.  I had no idea that a) my dad was a porn addict and b) his choice of perverted books were not normal.  So, the message she sent me was confusing.  Didn't all parents watch porn?  To this day I didn't know if she thought I wanted to be a porn addict like him or if I was perverted because I read one of his books that depicted rape?  At the time I had no idea what any of that was.  But then I grabbed one of his books and found out it had to do with sexual acts with children.  I put it back, feeling disgusted, and never got into his closet again.  And I wondered why anyone would have that book.  But rather than talking to me about, she did what she did best: accuse me of being a pervert.  
• Being sexually active as a teen means you're automatically a whore.  At age 15, I had a steady boyfriend, whom I dated for two years from ages 15-17 (never mind he was emotionally, physically, mentally, and sexually abusive).  Let's backtrack.....in grade 4, I kept getting bladder infections over and over again.  Then at age 15, the same thing happened.  The doc asked my mom to leave the room and asked me if I was sexually active, and I stupidly trusted the doctor and said yes.  He wanted to test me for STD's and found nothing...well duh.  I knew there'd be nothing to find, but he did it anyways.  So he told my mom and she said "You whore."  Then we went home and she started, in great mother fashion, she started throwing things around and screaming.  My dad came in and yelled "What's wrong?  What happened?"  She threw the bill at him and said "Look!  Your daughter is a whore!  She's having sex and was tested for STD's!!"  My dad's face turned dark.  "You fucking whore!"  This went on for awhile.  But I was with ONE guy....and we were in a long term relationship and maybe they should had been MORE worried that I was getting punched in the face, thrown across rooms, controlled to the point of losing all my friends, and was hurting myself to deal with it all.  Then, when we broke up and he started stalking me?  My mother told me "Just go talk to him!  He'll stop following you around if you just go talk to him!"  Never mind he punched me in the stomach earlier that day and I was scared to death of him.  
• I will put you in positions to get hurt, and I will do the same to your children.  When I was 14, my parents let me date a college freshman.  I didn't want to be alone with him when his dad wasn't home, but my parents kept telling me it was okay.  So he raped me out of my virginity.  Then let's add the countless parties they had when I was a kid, with drunken truck drivers and all sorts of messed up drug-addicted trash.  They never looked in on me and never cared what I was doing.  As a child, I was horrified to go to sleep alone while they had friends over.  I was obsessed with fear that someone would come into my bedroom to hurt me.  I have no idea why, I just knew it was a strong possibility.  Then, years later, my mother allowed a male stranger to go into the bathroom to use the toilet with my son taking bath in the same room, inches away.  Narcs are notorious to be loose on their parenting in these areas.  They don't care if you get hurt.  Maybe they want you to get hurt so they can blame you when you admit to what happened?  As it did a few years ago when I admitted I was raped at age 14.  My mother told all her friends "Oh god, she was sleeping around with everyone back then!"  Which was stupid, because what did rape have to do with that, even had that been true?  It wasn't, but wtf?  Let's also add onto this one that I also know I was molested by a family member, and my parents forced me to hug him at every single holiday, knowing I was scared to death of him.  They'd all laugh and laugh when I'd run away from him at 90mph and he would chase me.   And I am sure my mother knows he did this, but is choosing to ignore it, because she doesn't want it to be true.
• Sleeping next to your children of the opposite sex makes you a pervert.  This is one she's said to me (which led to the reason that I sent her that anniversary card).  And then she told me she said the same thing to her husband when he slept next his own girls.  Though, I will say, she made his all about the fact that he was sleeping next to his girls in his underwear.  But when I did it, it was the fact I was next to my kids at all.  Never mind she slept next to me as a kid, IN HER UNDERWEAR, but we're the same sex, so that's okay, right?  I just wonder what happened to make her believe this POS idea.  
• Everything makes you either a whore or a pervert.  Wearing too much makeup.  Wearing clothes that may show off things.  Anything sexual, ever.  Etc. etc.  She's obsessed with these things and probably feels that way herself, so she wants me to feel the same as her.  Everything is about sexual repression, and shame with her.  Which I think reflects the narcissist's 5 year old ego very well.  Children of narcs grow up with some fucked up beliefs about sex, with their mothers either being totally repressed or the complete opposite.  This is due to them reacting to their own mother's abusive views on sex, and that's due to their parents' abusive views on sex, and so on and so forth.  So, if your mother tried to give you her fucked up views on sex and now you're suffering?  Try to end that LOOONNNNGGGG cycle of fucked views being passed onto their children by being open and honest (and totally age appropriate) with your own kids.  This topic always brings up shame for most of us children of narcs, due to our mom's behavior or beliefs.  But work with a therapist or some work on your own to try to change how you view this topic and see if you can find some healing to undo the damage your mother has done to you so we don't keep (inadvertently) pass it onto our kids.  

I could probably dig into my memories and find some more (like the time I thought it would be funny to pretend like the kids I saw in a movie who were peeking at their parents having sex, yet it was pitch black & I couldn't see anything at all and my parents weren't even having sex, but my mom saw me and called me a...........you guessed it, a PERVERT!).  But you get the idea.  My mom is horribly sexually stupid and tried to pass her dysfunction onto me.  Yay.