Now, if that's the way you feel I think you are a horrible person. And if you call yourself a feminist and feel that way, then I think you are a complete piece of shit.
Ashley Madison is a safe space where people, particularly women, can go if they are in a relationship and want another partner. And that safe space is needed - and isn't only or even mostly used by "cheaters."
Here's what happens when a woman who is in a poly relationship goes on OkCupid or a similar "traditional" dating site:
1. She gets called a "slut" or various other similar things.
2. If she and her partner are not open about being poly for career or relgious reasons she risks someone from her life finding her profile and ruining her life.
3. In some countries she risks jail if it comes out if she's poly. In other countries she faces the death penalty.
On a site like Ashley Madison none of that happens. Guys there have to pay for their membership with a credit card and it's made very clear that if they are shitty to a woman on the site their membership will be cancelled and they'll still have to pay for the rest of the year. This means they are very, very unlikely to start slut shaming her.
Beyond that only paid members can find her profile, and they are not going to out her at work or to her family, because they'd have to admit they were there too and then either out themselves as poly or in some other way looking for people who are not their public persona profile.
Now, let's look at another type of situation. Say a woman loves her husband and her marriage is mostly good but for whatever reason her husband has no interest in having sex with her. Maybe he's impotent or on anti-depression medication that has killed his libido. Or for some other reason she does decide to have an affair. You know what two super likely outcomes are if she's outed by these hackers:
1. Her husband beats the shit out of her.
2. Her husband rapes her.
So, yeah, isn't it silly that many women may be beaten and raped.
Here's another example of how the site is frequently used. A gay man and a gay woman live in a country or culture where homosexuality is greatly frowned upon or else illegal. Coming out of the closet would put their lives in danger. So, they marry each other with the understanding that each of them can be with members of the gender they actually prefer.
If they get outed from the hack they could at the very least lose any and all contact with their family and friends. They could easily lose their jobs. And they could be beaten up, jailed and sentenced to death.
Point being if you are gleeful about the hack you don't have respect for people in the poly community, you support women being beaten and raped and you support gays and lesbians being beaten and killed.
I hope you feel fucking good about yourselves.