The general concept of a priest is a mediator between God or gods and man. A priest within a social context "is a person authorized to perform the sacred rituals of a religion, especially as a mediatory agent between humans and one or more deities. They also have the authority or power to administer religious rites; in particular, rites of sacrifice to, and propitiation of, a deity or deities. "(Wikipedia/Priest)
In Egypt the king was considered the high priest, the mediator between heaven and earth. Heroditus states, No woman holds priestly office either in the service of the goddess or god, only men represent priests in both cases. However, "There is evidence for women who were priestesses of the Goddesses and even gods." (Pre-Ankh.co.uk) "Royal Priestesses, Women were also permitted to play a role in temple rituals. Most priestesses served the goddess Hathor. Other women priestesses worked in temples as dancers, musicians, singers and acrobats. The most important priestess was known as the Gods Wife of Amun. This woman was usually the daughter of the Pharaoh. She held a very high position at court, and performed important rituals to honor the god Amun." (iKnowthat.com)
"The Greek pantheon includes both gods and goddesses and that, with some notable exceptions, the cults of male divinities were overseen by male officials and those of female divinities by female officials. The demand for close identification between divinity and cult attendant made for a class of female sacred servants directly comparable to that of men overseeing the cults of gods." (press.princeton.edu/s8368.html)
The Cananites who were neighbors to the Israelites had both male and female priests, "The priest or a male member of the community represented Baal. The priestess or a female members of the community represented Asherah. In this way, God's incredible gift of sexuality was perverted to the most obscene public prostitution." (thattheworldmayknow.com)
Each Roman Family had its sacred fire which was always kept burning. The father was responsible for this which was handed down to his son. In addition, "Ancient Rome, the Vestals or Vestal Virgins ... priestesses of Vesta, goddess of the hearth. The College of the Vestals and its well-being was regarded as fundamental to the continuance and security of Rome. They cultivated the sacred fire that was not allowed to go out. The Vestals were freed of the usual social obligations to marry and bear children, and took a vow of chastity in order to devote themselves to the study and correct observance of state rituals that were off-limits to the male colleges of priests." (Wikipedia/Vestal Virgins) Caesar was considered the the chief priest with other male priests who cared their worship of other gods.
From the above we can see that the pagan world had both male priests and female priestesses. As far as they could see, why not female priests?