BF/GF doesn't mean a thing to the Navy, but for husbands and wives there is a policy called "spousal co-location". They usually do a very good job of finding billets in the same geographic area (around 50 miles seperation at most) for spouses. They also usually stagger the rotations so that one person is at a deployable unit (ship, sub, squadron) while the other is on shore duty. This is especially important when children are involved. With either single parents or dual military couples, they are required to fill out Dependancy Care Certificates stating who will care for the children during short-term and long-term absences. This form must be signed by the care provider to confirm they are actually willing to follow through.
If the Navy is stations both sailors to deployable units, and the parents are unwilling/unable to find someone to care for their children for long periods, the Navy must discharge one of the parents (needs of the Navy decides who goes).
Back to your original question, BF/GF are at the mercy of the detailers, husbands and wives usually get stationed near each other, BUT don't count on it for a full career. There will be stretches of seperation, possibly 1 or 2 assignments in 20 years.
HOPE FOR THE BEST, EXPECT THE WORST.