Not if they really liked you wrote:
The majority of my college buddies are married without kids or are in a relationship. That means they hang out with each other as couples and I only get invited once in a while to be wheel 2N+1 (out of pity, I suspect). Anyone else know that feel?
This is only true if your “friends” really didn’t like you that much and you were always more of a hanger on rather than actually being close. As you age you let the unimportant relationships drift away, and get a smaller circle of close friends. Having a single friend is not unusual, difficult or a burden to invite along or hang out with.