A viciously sexual predatory beast. Always has the urge to have unprotected sex, and carries it out between 13 - 666 times a day, each with a different woman. A virgin girl would lose her innocence in mere seconds, just through plain eye contact with one, and girls as young as 2 are rumoured to have had to take [contraceptives] after being in the same room as one. On the off-chance you happen to involve in sexual activities with one and they do use a condom, that will still not be enough to protect you from [the might] of a horny [impregnator].
Sometimes (rarely) people with testicles are also referred to as impregnators for their ability to impregnate, however it is inappropriate to use it in this manner and generally only older people will use it like this.