Is it okay for me to skip my dad's wedding?

This would be his third wedding. He's kind of a s|ut. Before anyone asks I wish I was trolling.
His first wedding was to my mum. They had 4 kids. He up and left us with no warning and didn't contact us until like 2 years after he moved out, despite living literally half a dozen blocks away from us the entire time. No child support, mum provided for all of us on her minimum wage salary as I was 12 and the oldest so I couldn't work but as soon as I was 16 I had to get a job to help support us as dad also cleared out their joint account before he left. The youngest of my full siblings was 3. In the time he took care of us he was an angry drunk who psychologically abused all of us, ranging from calling us fat/ugly/stupid to actually taking the door off my room when I turned 10 (I was a very early bloomer and he didn't want me to be alone in my room with anyone).
We found out in the time he didn't speak to us that he had been cheating on mum with a coworker and she was pregnant with a boy (his third son) which is why he left.
When he got back in touch he was remarried to a woman who was not the coworker, who had 3 kids of her own and they had another 2 kids together (so grand total of 10 kids now). I was 15 when this happened and I introduced dad to my girlfriend (I'm a lesbian) and he stopped speaking to me.
After he broke up with this woman he got another woman pregnant. I don't know many details of this one, except she had a girl.
They divorced a year ago and he is now remarrying a girl who was literally in my year in high school. He is 54, both me and her are 20. She is pregnant. He invited me to the wedding.
He says he knows we had our fights (not a fight. I told him I had a gf and he literally got out of his chair, left my flat, and blocked my number and social media. There was no discussion.) but it would mean a lot to him if all of his kids (11 plus another on the way) were all at his wedding (no mention of his 5 baby mothers) (except the one he's marrying). He says he wants me to get to know all of my siblings, meaning the 7 step and half siblings I either haven't met yet or only know in passing. He says as the oldest it's my duty to bring all of us together.
I know they can't help how they were born, and I don't hate my siblings, but I also really don't want to deal with dad. The second oldest is my 15 year old full sibling. Of my 3 full siblings 2 have said they want me there and the other one said she doesn't care (she's 13 she pretends not to care about anything) but I think she'd appreciate it.
However, I'm not sure I can move on from the hell he put all of us through as a collective generation of this family, but also how he reacted to me coming out specifically. He said I could bring a date, but specified a guy (exact phrasing was "you can have a plus one in case there are any nice boys you want to introduce me to")
What do I do? Can I justify skipping the wedding when my siblings want me to go? Do I treat it as a high school reunion as my gf suggested? (she was joking but the attitude carries over) Do I suck it up and smile for one day or do I give my dad hell?

Personally I'd be wary about coming anywhere near him.
He sounds horrid, habitually inconsiderate and very hostile towards other people based upon their private lives.
I'm skeptical as to his motives in gathering all his progeny together for the wedding.
Either he's trying to rewrite history or give his new wife a misleading impression of him and his commitment to family values.

Sorry to hear of this. The best thing in my OPINION, is you know the answers already. You know what is best.
You may want to investigate trauma treatment, go on google, look for different therapies, and you will find a good healing technique.
That s what i would do.
Tallk therapy ok but angle of approach/healing very slow, depends which belief system of healing you go to.
good luck

your life seems chaotic but it’s your choice he put you through so much and you will never forget about it but think thoroughly about what you want to do.

I mean, I'm almost certain he has some sort of ulterior motive. I don't trust him as far as I can throw him. He was bordering abusive growing up and I'm a firm believer that people can change, but not that much.
However, I care for my full siblings very much and I want to give my half and step siblings a chance at least, as it's not their fault who their dad is. I've never actually met any of them. I think I saw one or two of them in passing, or in photographs, but that's the extent of it. The could be really great kids. Or they could be awful.
But I am nervous about being around dad again, because, like I said, there's no way there's not some sort of hidden agenda here. Maybe just like a pic of him and all 11 kids so he can brag about how great a father he is.
You know what the most ridiculous part of this is (even more insane than the 11 kids thing)? Dad is a therapist. He currently has a private practice and is doing a qualification in family therapy. I really really wish I was joking.

I would tell him to f*ck off, he doesn't deserve any family.

I'm not sure. My full siblings want me there and it might be nice to meet my half and step siblings in person. However, it could turn out that they're all awful.
My gf thinks it might either be his attempt to apologise or my method of getting closure. She thinks it might be good for me to go, but she's also said it's up to me because she knows about everything he put me through when I was a kid.
I think if I did go, it would be solely for my siblings, not for him.

I know he's a horrible person, but my gf thinks this might be his way of apologising. And while I don't like him all that much, I do care about my full siblings, who have asked me to come, and mum has said it might be a good way to meet my step and half siblings. It is a bit weird that I have people I'm this closely related to who I've never met.
If there were no siblings involved then I wouldn't even question not going, but this is giving me pause tbh.

yeah your half siblings and step siblings might be awful or they might be nice, you won’t know really unless you go. you also don’t want to live in regret and ask yourself what would’ve happen if you did go. it’s entirely your decision and what you feel is important for you and how you’re feeling on the day. hope everything goes well x

Well if you do go I advise against getting close to him and avoiding making him look good. Bad people don't deserve nice things.
Just talk to the siblings if that's your only desire.

Anyway I've really taken against him so theres my advice.

Stay away. He owes you everything and you owe him nothing. He’s toxic and a completely awful father. Don’t feel guilty about doing what’s right for you.