In theatre ‘The Fourth Wall’ is an imaginary wall separating the audience from the action on the stage.
Lovers looking at the camera as they fuck, even playing to the camera – acknowledging that they have an audience – has the effect of “breaking the fourth wall”. To involve your viewers more deeply in the intensity of the actual fucking, it’s better that you pretend the camera isn’t there.
This can heighten the voyeuristic experience of the viewer.
Then again, maybe your intent is to emphasise the amateur nature of what you are doing – that you are filming for your pleasure, and watching yourselves. You know it will be watched. It turns you on.
The camera is then one of the characters.
Whatever works, it’s your movie. So long as you understand the different effects on viewers.