该句语法上有些小问题.
整个句子的重点在"I'll marry you",但有个"we are still togeter after graduaing from university"的前提；
如果彼此都知道是“大学”毕业,"from university"可以省略,这样更加简洁、地道.
下面的翻译较为合理：
After graduation（或者after graduaing from university）, I'll marry you if we are still together.