I wholeheartedly agree with what has been said about aggression and females - unlike males - pairing together. I'd like to add something about gender however.
First off, there are some species that can switch between male and female... there are females in species that may switch between sexual and asexual reproduction - eg. having male and female offspring after sex with a male... or having only female offspring "by herself". In addition there are species - like some earth worms- that are both male and female, and that "battle" during sex. The winner is the one that with one of his three penises manage to impregnate the other - who will be the "female" actually becoming pregnant.
Gender may also be determent by external factors. For some reptiles, the gender is determent by how heated the nest become in the sun. Warmer nests gives most/only females, while a few degrees colder gives most/only males.
For many insects - like bee and wasps - unfertilized eggs become males, while fertilized eggs become female. It's speculated that this happened first for spiecies living of scarce resources in very harsh climate. Imagine a female crawling around the desert, trying to find a suitable dung-heap. When she does - rather that trying to attract a male - she just start laying unfertilized eggs. These became males - males with whom she could mate (yes, incest). Her next batch of eggs - fertilized eggs - all became female For this species, the female would wander, while the males stayed put.
My second point is that not all animals are XX=female and XY=male. Birds - which are closely related to dinosaurs (or rather, is the one group of dinosaur that remains) - have ZW, which is "opposite" of ours... Two identical sex-chromosoms ZZ=males, while two different ZW=female. This is also found in some reptiles.
Some insects (look above) have XX=female and X0=male - ie. the absence of a second X-chromosome - like we would have in unfertilized eggs - gives males.
More at: Sex-determination system (Wikipedia).
So it's not always as obvious as we're used to how gender is determent.
For Jurassic Park, it's also likely that they intended to have some controlled breeding in the future, and since the bottleneck with reproduction are the females, it would make sense to start with - and have most of - them. They wouldn't even need live males (for very long), since they could go for artificial insemination or test-tube dinos.