Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 3
The whole mistake is not to have a clear and unmistakable direction that sex (whether open or masquerading as deep romantic affection) and this Yoga cannot go together. This notion 
of making sex help the sadhana is one that has been taken hold of by many under one form or another and it has always proved an immense stumbling block to all who indulged it. It ties the being down to the vital and prevents the spiritual liberation which is essential as the basis of the transformation of the nature. Even the higher experiences begin to get coloured with the sexual tinge and falsified in their substance.