There's a lot of things people look for in a prospective marriage partner, one of them being physical compatibility. Of course, relationships that are solely based on shallow factors such as s*x are bound to dissolve once passion fades and reality settles, which is why 50% of modern marriages end in divorce. But what about marriages in the past?
In the past, divorces were very uncommon. Regardless of how unhappy they were with their marriage, few women had the ability to leave their relationship. Not only were they blocked legally, but women who went through divorces were often shunned by their peers (for many misogynistic reasons). But there were some grounds in which women could successfully divorce their husbands and retain a degree of their reputation. That is, taking their husbands to court for ED.
In the modern day, ED is still a cause of marriage woes (though not as much as it was in the past thanks to the invention of viagra), so it comes to no surprise it was one of the only acceptable reasons for marriage annulments in the past. In fact, women could take their husbands to court for impotence. This was because, in the past, marriage was created by the church in part as a means for procreation. So being unable to uphold your end of the bargain (i.e. having ED) was considered a form of fraud.
Nevertheless, the past being the past, getting approval for an annulment still took time if it is proven that a husband is impotent. Firstly, in order to pursue divorce proceedings on grounds of erectile dysfunction, couples would be subjected to a Trial by Congress, in which the court was sometimes literally allowed inside the couple's bedroom to ensure that the claims of impotence were not falsified. Then, they would have to wait for approval, which could to take up to three years, and during that time the wife (and in some countries the husband too) would have to subject herself to many humiliating tests to prove that her husband's impotency was not her fault (like tests to prove she was still a virgin).
In the end, if the woman was lucky, she would walk away from her marriage happily divorced and free to be pursued by someone more physically compatible with her.