There could be a million reasons why you didn’t get a plus one. Plenty of people don’t add plus ones unless you’re married or in a longer term commitment.
1. I’d do some digging to see if other non-married folks have plus ones, not just you. But don’t be mean about it. It could easily come up in some kind of conversation.
1a. If it’s across the board for everyone, go. Have fun. Your boyfriend can still go. He’s an adult. He can find something to do during the ceremony and reception if you want to make a trip out of it. Or go solo.
2. If it’s literally just only you who didn’t get a plus one, you can decline and then reassess your friendship. Or whatever feels right to you.
Whichever it is, don’t make a stink about it.