Actually it was a highly symbolic and politically-charged gesture. This was a Florida gay couple who was invited, fighting the state's gay adaption ban that goes back to the time of Anita Bryant's anti-gay crusade, for those of you who know your gay history.
This is what Presidents always do: set the public example, show their support or opposition to issues by acts like these. President Obama cannot personally overturn Florida's state constitutional ban on gay adaption. But he can demonstrate his disapproval of it, and send a message to Florida voters to vote to rescind it, and to Federal judges that the Federal government would support the courts overturning it in these ongoing proceedings.
I can't complain about this very clear White House political statement while this case is being fought in the courts.