I think they have real sex but it must be like in all other movies where they shoot a scene, then it is cut, then they have to redo it...
So I think they fuck for real but in between there are cut scenes, etc.
Tricks can be: shooting from particular angles where the cock seems bigger, or helping the actors staying hard by sniffing some substance, etc.
I have never directed any porn scene, I only think this is the way it should work.