In a recent in vitro pilot study,  semen samples were exposed to cellular phone radiation (in talk mode) for 1 hour. It was concluded that the radio frequency electromagnetic waves emitted from cell phones may lead to oxidative stress in human semen, and that keeping a cell phone in a trouser pocket in talk mode may negatively affect spermatozoa and impair male fertility.

In another study involving 304 men it was noted that an increase in the percentage of sperm cells of abnormal morphology is associated with the duration of exposure to the waves emitted by the mobile cellular phone. It was also confirmed that a decrease in the percentage of sperm cells in the semen is associated with the frequency of using mobile phones.

In an observational study of 361 men, the use of cell phones decreased the semen quality in the men by decreasing the sperm count, motility, viability, and normal morphology. The decrease in sperm parameters was dependent on the duration of daily exposure to cell phones and independent of the initial semen quality.

Keeping your mobile phone in your pocket or between your legs while driving may cause infertility.
References:
1. Agarwal A, Desai NR, Makker K, Varghese A, Mouradi R, Sabanegh E, Sharma R. Effects of radiofrequency electromagnetic waves (RF-EMW) from cellular phones on human ejaculated semen: an in vitro pilot study. Fertil Steril. 2008 Sep 18. PMID: 18804757.
2. Agarwal A, Deepinder F, Sharma RK, Ranga G, Li J. Effect of cell phone usage on semen analysis in men attending infertility clinic: an observational study. Fertil Steril. 2008 Jan;89(1):124-8. Epub 2007 May 4. PMID: 17482179.
3. Wdowiak A, Wdowiak L, Wiktor H. Evaluation of the effect of using mobile phones on male fertility. Ann Agric Environ Med. 2007;14(1):169-72. PMID: 17655195.