While I was reading the series for the first time, i never noticed that Marco never paired up. And thinking about it now, I'm not bothered by it. As a clue others stated: not everyone pairs up, especially as young as the characters were. Marco had other things on his mind at the time. It's kind of refreshing actually considering the length some authors go to to ensure their characters get a happy ending.
Though, I'm speaking as the one person in my circle of friends who remains unpaired.