Identification of that red and black bug
This looks like Graphosoma italicum (aka G. lineatum; it is a stinkbug):

Graphosoma italicum had been regarded as either a subspecies, or a mere synonym to G. lineatum.
G. italicum can reach a length of 8–12 mm (0.31–0.47 in). The body is almost round, with a large shield. The basic color of the upperside of the body is bright red, with wide black longitudinal stripes. The pronotum has six black bands. The antennae are black. Also the sides of the abdominal segments (connexivum) are red with many small black spots. The legs are black.
Wikipedia

I think you are right that they are mating. Here is another picture of the species mating:


ibid

(Despite it saying that the legs are black, they are clearly striped in some specimens, like in your picture.)
It's found in France, according to Honeyguide.