Thanks, Koala, for so kindly pointing me towards etiquette sources. 
I'm 'hung up' on Debretts only because you named it (yes, I know, along with others) and quoted from it to support your argument, then seemingly contradicted yourself by saying you didn't care about or take notice of it.
How the heck would I know how people feel about it? Well, let's see.
DP and I have discussed the weddings that one but not the other of us has been invited to. So I know how he's felt about them and vice versa.
Then, let's have a think again, I've discussed this with friends who've been in the same position, so we know how we feel about it.
And I've discussed it, after the event, with some of the people who invited me but not DP, or vice versa, to their weddings. So, again, we know each other's thoughts on it.
Unless we've all been furiously lying to each other, of course.
Friendships preserved and etiquette manuals of any description not needed. Although I love a bit of Emily Post.
Anyway, I thought you were done. 