We have featured some of the erotic works of French painter Octave Tassaert before (here). This one, of two lovers, probably dates from around the same time, 1860, just a few years before he stopped painting altogether,descended into alcoholism (which damaged his eyesight) and committed suicide by gassing himself (as did JW Godward).
What is unusual about it is that it is a painting. Most erotic art of the nineteenth century was done (often anonymously) to illustrate limited edition erotic books, so was more usually done as lithographs which could be easily reproduced. Mainstream (rather than erotic specialists) artists who did produce erotic work tended to do so as drawings or sketches (occasionally from life, such as with Turner). To find a fully rendered, explicit painting by a well known artist (as he was at the time) such as this, is unusual, therefore.
One of the main themes of Tassaert's paintings was the lives of the poor so it may be that our copulating couple are ordinary people rather than than from high society. Whatever, the lady's expression indicates that she is enjoying the experience.