The 41-year-old man is accused of repeatedly harassing two female prisoners between December 2011 and December 2012 and taking advantage of his position to persuade them to perform sex acts with him.
At some point he coerced one of the woman into having intercourse, after having previously got her to perform oral sex on him.
The two women accuse him of having “used his authority” to get them to comply with his demands, but did not accuse him of sexual assault or rape.
The man is pleading not guilty.
However, judge Andreas Böhm recommended that the charge be amended to rape, saying "if the incidents took place as described, then he is guilty of rape".
If found guilty of coercion he would face up to three years in prison, but if found guilty of rape he faces a sentence of up to ten years.
The allegations first emerged in the autumn of 2013. A female colleague of the guard filed a complaint after hearing about what was going on.
After an investigation the man was suspended from his job in November 2014.