I wonder... Do the minotaurs in 4e have females of the species? Or do they just use humans to reproduce themselves? Or are they born in the void and come out of nowhere?
I've never seen any female minotaur in any of the illustrations...
Yes, there are female minotaurs.
Nothing in the race description specifies that the race is male-only (the way satyrs are explicitly male-only and hamadryads are explicitly female-only), ergo there must be female minotaurs.
It's just a hypothesis, but based on the people I've known who've played minotaurs, the reason there are no images of female minotaurs is that very few people play them. Also, a quick Google image search for 'female minotaur' yielded a relatively low total number of images, and most of those were... fetish-oriented... rather than heroic. There aren't a lot of minotaur images in 4e books in the first place, and the artists probably opted for the traditional depiction.
Player's Handbook 3's race entry for Minotaur depicts one male and one female minotaur. Also note that they clearly specify male and female name examples.
Player's Handbook 3 p. 102 depicts a female minotaur in the Runepriest section.
Primal Power p. 109 Warden paragon path Emerald Guardian depicts a female minotaur.
Dragon Magazine #389 p.32 Winning Races: Minotaurs features a female minotaur. The article also discusses (albeit in passing) marriages between minotaurs.