Why does the Party permit couples to marry but discourages love?
The answer to your question involves the cold, practical use of marriage versus the war, emotional uses of love.
Allow people to marry is valuable to a society such as the one "Big Brother" wanted, especially from a 50's sort of standpoint. Married people will have children, and the party had final say in who got married.
Trying to eliminate love from the equation of marriage was a way to preserve party loyalty. If you have absolutely no love (or really, any attraction at all) to the person you're allowed to marry, there is no chance that you will care more about that person than the party, which would create a conflict. You will also be more willing to monitor that person and turn them in should they "think badly."
Aside from creating future party members through the creation of children, the children also can act as little spies. If the kids don't care about their parents, they make great agents to monitor their parent's actions.
In short, the Party allows people to marry to confine their sexual activity to one person, to create children that will strengthen the party, and to ensure that no-one who actually loves each-other will end up together.
In 1984, the Party encourages couples to marry for the purpose of procreation and such marriages are arranged by a specially-appointed committee. (This is how Winston and Katherine came to be married, for instance). In contrast, the Party discourages love because it is afraid that such personal emotions will distract attention from Big Brother. This is best summarized in Part Three, Chapter Three, in the following quote from O'Brien:
There will be no loyalty, except loyalty towards the Party. There will be no love, except the love of Big Brother.
In other words, if two people fall in love, their attention and interest is focused on each other, not on the government. If they are not focused solely on the government then it becomes impossible for the Party to control and manipulate them.
In Oceania, then, falling in love is a political act which has serious consequences for the Party's power.