Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 3
That [indulgence in sex-thoughts] is just the thing that ought 
not to be done. It would be merely a gratuitous increasing of the difficulties. For the spiritual endeavour is part of your nature and, if it is difficult to get rid of the sex-impulse, you would find it still more difficult to do without the spiritual life in you. Sex is the one difficulty in its way that is hardest to get rid of, because it sticks and returns, but one has to be more persistent than the difficulty – there is no other way.