Why must there be a versus?

Can someone tell me why it is that the single people tend to get left out of things...such as being invited over for dinner? At least 3 couples have invited some friends of mine over for dinner since the husband has started working at the church. Don't get me wrong, I'm happy for them. I just don't understand why I get looked over. Is it because single people make couples uncomfortable? Do they think it would be awkward to have an odd number sitting at the table? Or is it simply because we lack the common connection of marriage and kids? I don't know why this has hit me so hard today, but I had a minor breakdown this afternoon. Even though I know it's not intentional, I feel left out and somewhat invalidated. I'm searching for community but I'm not sure where I fit. Most of my single friends are in college and the friends I have that are closer to my age are married, many with kids. Is there a way to get over the societal norm of single v/s married? Sometimes I wonder.