I'm so sad to hear that your SO reacted that way when he met your family.

Yep. He's generally an extremely respectful and accepting guy, but the situation here is that both he and my family are immigrants but his English is much, much better despite being in the US 1/10th the amount of them. I know it's because even though they are immigrants he is from a highly developed Western country with a language much more similar to English, plus he started learning it as a child vs. my parents who learned it in their 30s. Sounds like I should be okay with because a larger cultural and linguistic difference justifies it, right?

No, because that difference is what makes people unable to take them seriously. The reminder of that hurts.

The worst part? They actually have excellent English, better than that of a lot of native-born Americans, but are socially inept and can barely speak at all to people they don't know. Having an accent of a culture known for horrible English proficiency doesn't make anyone believe that though.

I guess I just don't value "respectability politics" (not trying to imply that you do!)

I generally don't, but sometimes when it comes to your own family it you start wishing they could act more "respectable". Even though I myself don't disrespect people with X qualities, I have to acknowledge that the opposite is also the norm. I mean, I believe that in time, society will start valuing a wider range of people, but will it happen by the time my SO's family meets mine?

If someone tries to position your family member's accent (or whatever else) as a bad thing

As an amateur linguist and someone who has studied foreign cultures/languages way too much, I have encountered many, many horribly annoying accents from every race and am very sympathetic with people who don't like some accents. But yeah, using someone's accent to label them as actually inferior and being condescending about it is definitely going too far. I know this sounds silly, but sometimes when I'm angry/threatened I'm tempted to talk in an annoying, rough accent to piss the person off even more (if it's a stranger who doesn't know how I actually sound). By "annoying, rough" I'm referring to very deep, regional US accents...

and I hope I'm making it clear that this is so not meant to be a critique of you for your feelings, but rather it's supposed to be a critique of whatever jerk is behaving in a way that could make you feel this way.

I totally understand :)