>"marriage and family go against the central theme of these characters".
Just to give it a little more context
>Superman
Morrison, Millar and Waid believed that Clark Kent was just a character played by Superman, and that Superman is incapable of loving a human romantically. Therefore, if Superman falls in love and marry someone, or Clark is real, or Superman is able to fall in love, and either way, they see this as "going against the central theme".
Or they SAW this, since Morrison and Waid change their minds eventually and only Millar things that nowadays.
Busiek believes in Clark, so his reason to prefer a single Clark is to make him bang Lois, and Lana, and Loris, and Lucy, and Maxima, and Linda, and Ice..., instead of just Lois.
>Spider-Man
"Being Spider-man is suffering, por lo tanto, he shouldn't have a loving and supportive wife, because that would make his life easier, while having to lie and hide secrets from girlfriends would make his life harder."
This is their reasoning.
They also like to give Peter a harem, but since Mephisto the only one having fun with a new boyfriend every year is MJ. Peter is banging midgets.
>Batman
Similar to Spider-Man. Having a person that loves him while he loves her back would make his life less sad and grim. So they don't want that to happen.
It is really a case where creators think that "character progression" shouldn't happen if that means a changing what they believe is their theme.