https://pmc.ncbi.nlm.nih.gov/articles/PMC3564677/
Eleven of the 27 subjects (41%) produced pre-ejaculatory samples that contained spermatozoa and in 10 of these cases (37%) a reasonable proportion of the sperm was motile. In every case where an individual subject produced more than one sample, he either did or did not have spermatozoa in all of his samples. In other words, it was never the case that a subject sometimes had spermatozoa and sometimes did not.
It has been suggested that any sperm in the pre-ejaculatory fluid must be the result of a previous ejaculation and that men who practice withdrawal should pass urine prior to coitus in order to wash away any residual sperm (Withdrawal Method. Planned Parenthood, 2004). However, in all cases in which we observed sperm in pre-ejaculatory fluid the urethra had, of course, been washed with urine on multiple occasions after the last ejaculation, and therefore the contamination of pre-ejaculatory fluid must have taken place immediately prior to ejaculation.
It would appear from our study that some men repeatedly leak sperm in their pre-ejaculatory fluid while others do not.