This study (N=830) says " There was no difference between tamsulosin [(Flowmax)] and placebo or alfuzosin with regard to the occurrence of decreased libido or impotence. In addition, there was no significant difference in the change in sexual function score between patients treated with tamsulosin and those treated with alfuzosin. Compared with patients receiving placebo, there was, however, a significant improvement in total sexual function score in patients receiving tamsulosin (p = 0.042)."

http://www.ncbi.nlm.nih.gov/pubmed/10473995