Cyllarus was speared to death during the battle between the centaurs and humans at a Lapith wedding feast. Supposedly, the centaurs followed their true natures after imbibing too much of the host's wine and proceeded to abduct and rape the unwilling bride and other female attendees. This, of course, was met with major disfavor and all out battle ensued.
The battle has since gone down in mythology as proof that the centaurs were uncultured and wanton beasts preying on hapless, sensitive humans. I fear it is just more pro-human sentiment carried to prejudicial extremes.
What is of note here is the depiction of Hylonome who many considered the first individually mentioned female centaur in mythology. Where the female taurs were before this is not known. Did she spring up from one of Cyllarus' ribs? And it is odd that she is mentioned in this context because, if Cyllarus was mated to her, why was he off carrying on at some wedding? Just more proof of what indecent brutes those centaurs were, eh? Right.
After Cyllarus dies Hylonome, grief stricken, takes her own life. I suppose this romantic doomed-love story telling prop was used to shore up the idea that heterosexual relationships were somehow more valid than Cyllarus just hanging out, as it were, with his male cohorts.
Since the belief in the repulsive nature of male-to-male relationships was to some extent expanded over the centuries by ignorant and prejudiced historians and mythologists, providing a centaur with a female partner kept the real ribald shenanigans out of print. Equines, by Grecian standards of culture and beauty, were just more acceptably noble creatures than goats so they were made to act accordingly.
Of course, without females the race would never have occurred except in broad magical conception. The idea that raped humans could bear half-horse offspring is a bit too hard to swallow. If it were possible, the human mothers would more likely have died in childbirth and the offending offspring would have been slain. That doesn't bode well for race proliferation, does it?
Mythology is very catholic in its retelling; it absorbs new variations along the way and gains new purposes that weren't part of the original tales. There is much room for open debate as the written record is slight and can have multiple interpretations.
So it is that centaurs were at first depicted more like what are called boytaurs these days, guys with two sets of legs and genitalia. Later they became a more upright and beauteous blending of the human torso with the equine trunk and legs. And, in an effort to make more rational sense of such a creature, humans began to ascribe to it their own believed more admirable attributes. One of those, no doubt, is the addition of female centaurs with Hylonome's appearance in poetry as a doomed lover.