I think you can find the answer in an Australian interview with lead actress Katia. It's stated that the male actor is from the adult industry and Katia says this:
"Possibly due to her theatre-background, Pascariu seems to take on any challenge thrown at her.
'I told Radu I'm totally OK. I have no boundaries or restrictions if I find it OK for the script, for the story. My body is there to be part of my work. And we did some research, which was very fun! We were like kids discovering a new world. We needed to see home-made videos. And Stefan told us that even they as professionals are asked for videos that look more home-made. It's very trendy now apparently. It was really useful because we got some ideas that we copied. For example we saw a video where the woman was wearing socks so Radu said "Oh I want socks!". Also it was great how they created the space, the bedroom, to look so homey. I felt relaxed and safe. It was a small crew and everyone was very careful and respectful. And because Stefan was a professional I felt I had to level-up. AND it was shot in our producer's offices! Low budget, you have to adapt!'
So, it can be understood that the sex was real.
