Why does the Party permit marriage but discourage love?
Three Newspeak Dictionary terms are applicable here:
ownlife: individualism and eccentricity; desire to do something for your own benefit. (i.e. hobbies, ownership of property, love, or any other Thoughtcrime)
good sex: sex for the purpose of producing children for the party; opposite of sexcrime
sexcrime: having sex for enjoyment or in the hope to create a family of your own
So, marriage is permitted only if it is state sanctioned. All children produced are for the party. Love and sex for pleasure are not condoned because they lead to ownlife. Ownlife is a thoughtcrime, in that it is individualistic. Again, the purpose of the state is to destroy individualism.