Very few. In fact, the disadvantages far outweigh the advantages in most places. Here is a list of advantages:
No unplanned pregnancy
Understanding your partner better (though this is debatable)
No sexism in the relationship
Being openly gay, particularly if you are a gay parent, is slightly trendy today (in some areas) so those gay people might have an easier time making friends or fitting in, though this happens only in very specific situations.
You can go to the restroom together.
Depending on the couple, you may be able to share clothing...in some cases, an entire wardrobe.