You forgot to mention his friends are jerks.
You have every right to be upset. Really, if it bothers you that much, if you're that upset over it, any kind of decent person puts your feeling ahead of his single friends. There's no reason for him to go; it's not like it's a job requirement.
He needs to respect your feelings. Yes, his friends will bust his chops, but so what? That shouldn't take precedence over his relationship.
OP do him a favor and break up with him, will you please?
This answer is a perfect example how radical feminism has completely overtaken the media. Guys are demanded to subordinate their own lives, interests, even their identities to the woman - who then complain there are no "real men" out there. But, of course, if SHE wanted to go to a porn convention (and yeah plenty women do go - it's another feminist myth that no women like porn) over his objections and feelings, he would be called a sexist controlling pig. Going to a porn convention does not equal cheating or even having sex. Hell, from the pics my coworker showed me there wasn't even any nudity except on videos and posters and he's been to many of them.
The boyfriend, if you by some slim chance read this, run, run very far very fast. Today it's porn conventions, tomorrow you'll be guilted for being an insensitive jerk for wanting to go to sci-fi or comiccon conventions, or, for enjoying watching sports or playing video games, or doing whatever else you enjoy out of life that she happens to not like. Or, just go anyway, after all, remember, women claim to want a man with balls who stands up for themselves, and you would be doing exactly that - on both accounts.
I'm kind of confused how feminism got pulled into all this. IMO, this isn't about a social movement... it's about two people in a relationship not communicating. "Porn convention" could easily be replaced with "dirty dishes" and it would mean the same thing. Something bothers me, my SO knows it bothers me and yet is unwilling to sacrifice not going to the convention/doing the dishes. What does this mean/should I be mad?
I think she does have a right to be upset if they had a conversation about how this activity makes her feel and he's like, "Yeah, I totally get it," and then disregards her feelings and does it anyway. It's the disregarding that's the problem.
If the shoe was on the other foot, she would be a jerk for disregarding his feelings, too. It really comes down to respecting your partner.
When someone disregards someone's feelings about something in a relationship that is usually a sign that the relationship is over. When I was married I was uncomfortable with my husband going out to bars late at night without me. He would leave the house at 9pm or 10pm not come home until the bar closed or later. He did this ever Friday and Saturday during the last year of our marriage. I told him I was uncomfortable but he dismissed my feelings and said he was not cheating on me. Finally when I confronted him at 4am one morning I found out he was cheating on me. When you are in a relationship with someone you need to respect your partners feelings. If you can't you do not belong in a relationship with that person. It has nothing to do with feminism it has to do with repsect.
He hasn't done anything yet, so there's still a chance to discuss this. Don't be upset, just tell him how you think people should treat each other in relationships and how his attending this convention is disrespectful to you. Usually you can find some common ground and perhaps he can convince you to his way of thinking. Keep an open mind. You may end up going to this porn convention together or watching porn together. If you find yourself in complete disagreement of your relationship expectations, you need to seriously consider leaving. You cannot be with someone whose concept of a good relationship is much different than yours.
Seriously, what kind of man would do something like this? Especially after 3 years. Tell him to go with his boyfriends and don't forget to mention that you won't be there when he returns!