I would need to know who the hosts were. If it's your SO's sibling or parent, that's one thing -- if it's his second-cousin, among dozens, I would cut them some slack. The precise status of our own relationships looms very large to us, but we have to face the fact that they just aren't that central to everyone else's lives, and they may not keep as close track as we'd wish.
That is one reason we have social (never mind legal) statuses including marriage and engagement; besides what they mean to the participants, they give clear signals to everyone else.
Today, we consider live-in boy/girlfriends to be in the must-invite SO category. But believe it or not, your cousins may not remember who is and who isn't living together, especially if they are in another city. For example, my daughter now lives with her (very serious) boyfriend. But it's not like they sent out announcements about it, and they are in Israel. So when my cousin in Oregon emailed me for our kidz' current addresses so she could send bat mitzvah invitations, I realized she might very well not even know this guy exists, much less the status of the relationship. I decided to tell her and suggested she add his name to the invitation (very safe as they can't come anyway!). If I hadn't, and if they lived closer and my daughter decided to come, I think she would have been fine to call them and ask if she could bring him. I don't think it's even their living together that would suggest that she do that; it's that they are very close to getting engaged. Her asking for them to include him would very definitely be seen as a signal of an imminent engagement, not as a statement on the current status of their relationship as a must-invite-together couple.
While I do agree that cohabiting couples should be invited as a unit, bear in mind that people who do not do so aren't necessarily disrespecting your relationship even if they do know that you are a serious couple and live together. They may know one of those more rare, but not nonexistent, couples who do NOT wish to be considered a social unit -- one reason they AREN'T married. So your choice not to marry each other may be taken, incorrectly but not necessarily meanly, as a signal that that is how you feel, too.
As to rude strangers -- that's all they are. Ignore. (My husband has a couple of times been taken as my father, as has my brother, even though they are respectively 2 and 4 years older than I am. How do you suppose that made THEM feel!)