Thought it would be simple

Hello Feli. You cannot expect others to honor and respect your relationship when you cannot do so in return. It matters not if you met the bf once, he is someone important to your mother and she's put you on notice. I think you should also ask yourself why your Dad would not bring his lady friend of 15 years around you and others. If you want to end the cycle of toxicity, everyone is going to have to face and break old habits.

If it were me, a wedding is different than a normal family function and ettiquette deems anyone regardless of how many times you've met them or how long they've been together, anyone who is in a relationship should get their SO as a plus one. So for me, I would invite both your mom's boyfriend and your dad's lady friend, and then let them decide from there. But that's me.

We have a situation where a newly separated couple will both be attending our wedding (family members) and we're not allowing either of them a plus one, due to the fact that it's still so new, and it would cause drama.

I think it's really awful that mom is prioritizing her 6-9 month boyfriend over her son. It seems pretty selfish and manipulative to threaten not to come to the wedding over this too. OP, if you want your mom and extended family there, it may be easiest to just invite the boyfriend, but I'd recommend having a conversation with your mom about how it isn't right of her to put you in this situation.

I 100% agree with this. I can't even fathom a little bit telling my child that I am prioritizing someone I've been dating for less than a school year over them. Its beyond me that anyone would threaten to not go to your wedding over this. It seems absolutely insane. I think you definitely need to have a "come to Jesus" conversation with your mom about why you don't want him there etc. I would also mention to her that you are extremely hurt that she's refusing to come to your wedding over not being allowed to bring a boyfriend with whom you don't have a relationship with.

Kudos to your dad for not putting you in a situation where you would have to make the choice of whether or not to have his SO there. It sounds like your family functions have, for the entirety of their divorce, been run as though your parents divorce is a non-factor. Your parents don't bring their SOs around etc. Which their choice. May not be the one anyone else would have made, but if it works for your family, than cool. I would discuss that in particular with your mom.

I'd try and be unemotional about it and not get too over worked (something I struggle with!) and just be very frank and let her know that this is truly hurting you. There's not easy solution when these kinds of things come up. But at the end of the day it's your wedding and no one should make you feel as though you aren't meeting their expectations. The only expectations you have to meet are yours and your partners.

I feel like people saying it's rude or disrespectful to not invite your mom's boyfriend of 6-9 months do not come from a divorced family or understand the complexities of one. While I do think it's a little strange that your dad doesn't want to bring his "lady friend" of 15 years, I don't think it's fair of your mom to be taking the hardline stance she is. Had she been dating her boyfriend for a longer time (such as 15 years) or made it very clear that they both intend to make their relationship permanent, then I think circumstances would be different. Sometimes people even have a track record of getting into relationships that don't last for more than a couple of years. It's completely understandable that you wouldn't want a new person at your wedding who might not be in the picture a couple of years from now. This is something that I wish your mom was more reasonable about and was not placing such a difficult decision on you, at least not without having a discussion with you about it to explain why she really wants him there and understand why you don't want him there. I also think that her boyfriend should have made more of an effort to get to know you if they're this serious, or your mom should have been facilitating you guys getting to know one another - this could have eased your feelings about including him if you were able to view him more as a part of the family.

I agree with other posters suggesting you have an open conversation with your mom about this. Unfortunately, you may ultimately come to the decision that inviting her boyfriend is the path of least resistance - if you do so, I would just connect with your photographer and ask to make sure her boyfriend isn't prominently featured in your photos. That way if they breakup somewhere down the line, you don't have to worry about him ruining photos.

The boyfriend should definitely be invited. This is exactly what happens when this etiquette rule is ignored. People get angry and for good reason. You can't disrespect someone's relationship while having them come to celebrate yours. It just doesn't make sense.

This family has been divorced for decades. I'm not sure why it's really relevant.

I guess you have two choices, invite people with their partners or go without having everyone there. I don't see another solution here unfortunately.