There's examples on both sides. It probably mostly depends on what the affair is supposed to be saying in the story. In the 50s, for instance, when conformity was so encouraged, there were a lot of stories for people longing for what they didn't choose, for naughtiness or exciting but wrong lives. Sometimes the passion is the thing people like, which again can apply to either the man or the woman. I've seen plenty of stories where the wife is the old battleaxe to justify wanting the guy to leave her for the heroine. When I watched soaps I didn't notice a big trend in terms of men or women being more at fault for affairs. It depended on the characters. Sometimes the audience wanted the new couple to stay together, sometimes they wanted the original couple to be together.
Things have changed, but there are plenty of remnants of the old traditions still around where the man is supposed to be the provider (even though in many marriages now the woman is not only older but makes mroe money) and the woman is supposed to be the constantly attractive sex object. The main advantage men had in the provider days was that they also had far more legitimate power in the world.