It's an interesting one isn't it, about how much blame should devolve onto the unmarried party to an affair?
Traditionally, both parties were considered equally to blame because marriage was a sacred institution, upheld by law, the church, employers, the lot, and anyone who transgressed it, married or not, was a sinner and a social nuisance. Anyone who had an affair with a married person, man or woman, had absolutely no doubt whatsoever that they were categorically doing wrong. The fact that the married person had problems in their marriage, was unhappy etc., was irrelevant - to commit adultery was wrong, for both parties.
As marriage becomes less of a public institution and more of a private arrangement, it becomes no longer the property of the whole of society. It also becomes no longer the duty of every member of society to uphold it. It's therefore unsurprising that inexperienced young people have no idea that undermining a marriage is wrong. Strong, simplistic taboos had their uses.