A hookup with Clara is just very wrong considering his Marriage to River, and the Love with Companion thing is definitely getting old now.
Now this is a whole different (and more valid, imo) argument.
1) At what point is the Doctor considered a widower? He did see his wife die before he married her. Is it after their last date?
2) This is subjective, but I can appreciate this argument more than the silly idea that the Doctor shouldn't have romances.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy