Should be Katharina Buchberger. She's in a relationship with the guy she has sex with. He's also one of the directors and founding members of the production crew.
It was his girlfriend(reason they actually went real and without any condoms). I chatted with him (cool guy btw) and he said all scenes were shot in two different angles: one simulated and one non-simulated. The real sex ones were filmed with just him and his gf at the time on set. He said they basically set up the camera there without any production crew in different positions and recorded themselves having sex, which according to him was not that difficult (they just had to do what they used to do
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These scenes were then merged that with the simulated ones). He did that because some scenes had other people in it (like the group one), so it would be harder to have actual sex in front of so many people.
He also confirmed that Unholly Ground was the only film he made with real sex (all others were simulated). Though in Talk to Me, Sweet Darling you can see that he gets a semi and a hard on in a couple of scenes.