As Anneza says,
Leslie was male and
Lesley female in the UK. The British name expert
Leslie Dunkling, who is still alive at age 88, is definitely a man. I remember he once wrote that he really disliked it when people misspelled his name as
Lesley because they were "changing his sex."
The spelling difference by gender never developed in the USA, and since the actress
Leslie Caron became famous in the 1950s most Americans have probably seen
Leslie as being more "feminine" than masculine, though there certainly are many men in the USA named
Leslie born before 1960, as you can see from the popularity chart on this site.
The last two or three decades
Leslie has been particularly common for girls in the Hispanic community in the USA, perhaps because its spelling yields about the same pronunciation in Spanish as in English and so is attractive to parents whose children will be growing up in a bilingual Spanish-English environment.