This is one great oddity. Porn movies have never been known for paying much attention to the music. There may be some exceptions, but most of the time royalty free library music is used or they quickly slap a few samples together. In 1987 German porn producer Hans Moser (alias Sascha Alexander) released an album with a bunch of original disco pop songs which he also used in his films, mainly the Foxy Lady series with Teresa Orlowski.
This video clip is directly from Foxy Lady 7. The Fire Inside is a typical disco pop song of the time and, all considered, not bad. It even includes a cool guitar solo. But I’m still not sure whether the whole endeavor was just a joke or did they really expect to hit the charts? The singers and musicians in the video remain anonymous. The singer goes by the pseudonym Larry Bonville and the female singer is credited as Madame X.