As a student of performing arts in film, it’s not really much of REAL sex. Although the actors are really naked. And seems to me, they are, it could be most likely humping on each other with their genitals touching each other.
The close up shots are definitely not a real sex but ofc it must look real. The long (wide) shots such as the boat sex scenes on bird’s eye view can sometimes be another actors having REAL SEX.
As a Polish citizen with lots of common sense I can tell you there was no extras having real sex, and the actors didn't either...It is a Polish movie, and lots of people invested money so this crap would be made, one of biggest Polish tv Channel was a partner and they would never have part in sth like real sex shown in a movie. This is not Nymphomaniac movie. It is a really so-so production, they don't even use vulgar words in it, even though the book is full of it.
People seem to not understand the basic difference between PORN and EROTIC movie: in porn there is hardly any acting, and there is real sex, in erotic movie there is a bit of acting, sometimes even good one, and lots of fake sex. 365 days is an erotic movie.
The writer of the book and script was selling strange stories about spontaneous erections and shit like that, but truth is if you make a movie about sex you gonna tell stories about sex to sell it. It's not like they would try to sell it talking about children Morrone has, or family life, or puppies and flowers...Somebody mentioned the actress and Morrone may be a couple...She has a boyfriend. And it is not Michele.