Are relationships between faculty and graduate students at different universities acceptable?
Short answer: generally yes.



Long answer: 

It is commonly understood that consenting adults are in general free to enter voluntary romantic relationships with whomever they choose, except for people over whom they hold professional authority. Thus, a professor may not have a relationship with her or his own student. That’s a big no-no anywhere I’m familiar with.

As for graduate students in the same institution but in other departments, whether a professor is to be regarded as being in a position of authority over such a student is a matter of interpretation; opinions about this seem to vary considerably, and I think it would be a mistake to characterize any particular opinion as being “commonly understood”.

As for a professor having a relationship with someone who is a student in another university, there may be specific circumstances where that would be problematic, such as if the student is applying for a postdoc or other position in the professor’s department. Even this doesn’t make the relationship taboo, it only means appropriate measures must be taken to ensure the professor isn’t exercising any decision-making authority over their significant other’s application. But generally speaking, other than those specific exceptional circumstances, this doesn’t violate any rules, and isn’t frowned upon by reasonable people.