No, they don't have separate male and female plants. Citrofortunella mitis (or microcarpa) is its Latin name, commonly known as Calamansi, or sometimes Kalamansi, along with lots of other local names (panama orange, philippine lime). They usually have a few spines spread about, but I remember reading about these a year or so ago - I recall something about very spiny versions of this plant in the Philippines, and much less spiny ones, and it turned out they weren't the same plant, although they had the same common name. Trouble is, I can't find the information I had then...and I don't recall whether the spiny ones produced fruit or not, or whether they needed to get much larger to fruit. You may have to allow the spiny ones to get larger to see what, if any, fruit they produce. Sorry I can't be of more help, if I come across what I read last time, I'll post it here...

Alunparfitt, were these grafted trees? Some of them may have been taken over by shoots from the understock.