Lets take a moment to appreciate the sheer stupidity of our protagonist. Your artist little sister draws some porn, and asks you to critique it. If you can't at least look at it professionally and tell her the most basic of basics here, how dumb are you?
It's like, it's not even a case of not wanting to talk about sexual things with women, because he literally goes on to show the same porn to two other girls. Because he can't tell her the dick looks weird and balls don't explode? That's weird, man. That's some cringe level of purity. I'd get it if his sister wasn't the one who drew the porn, but you know. She's past that stage bro, time to grow up a little bit.