Hugo Schwyzer says the act wasn't always seen as degrading:
[Sex educator Charlie Glickman] suggests that the AIDS crisis and the concern with safer sex was what made the facial popular. "Cum on me, not in me" was a popular sex educator slogan as far back as the late 1980s.
Ejaculating on a woman's stomach, however, usually meant that the camera wouldn't let the audience see the actress' expression. But if the male actor came on her face, the viewer could see two things at once: evidence of male pleasure (symbolized by the ejaculation) and the equally important sign that a woman's reaction to that pleasure mattered. With sex now so dangerous — and HIV particularly likely to be spread through semen — facials were relatively "safe." But in the era of AIDS, they were also compelling visual evidence that a woman wasn't threatened by a man's semen. In that sense facials were, almost from the start, more about women's acceptance of men's bodies than about women's degradation.
The Dish 