Is this the same guy you were marrying few years back? Or is it a new guy?

If I was invited to a wedding and invitation wouldn’t include my husband, I’d not go. And I would be shocked if my husband went to a wedding if I wasn’t invited. I wouldn’t stay with him. Even if you two aren’t married, invitation should say plus one and you should be able to attend.

I’d not worry about what his friends do or think. You have no control over it. But you have control over your own life. Particularly control over who you are in a relationship with. If after years of being with a person you are still not fully his partner and can’t even attend events with him I’d say it’s time to move on. I’d feel he doesn’t value you. If he doesn’t value you, why would his friends?