I've never heard of this website before, but it doesn't take a genius to work out what it is...
I'm so sorry, you've found this.
I never know what to suggest when I read a post like yours. Largely because I found something similar on my exH's laptop history (I'd asked him if I could use it and when I went to clear my own history, saw this site listed in the history).
I emailed him a link to the website and phoned him at work. He refused to answer the phone and a colleague answered it for him and said he was in the toilet.
I texted him and said if he didn't answer the next time I called, I'd tell his colleague exactly why I was calling.
He answered.
He lied. He said it was a pop up. When I told him about the login pages, he said it was a pop up he'd been curious about but hadn't done anything more than log in. When I told him I could see he'd been looking at profiles and messaging women, he told me it was a mistake...
I told him to pack a bag that night. Then I told everyone why I'd kicked him out.
Then I discovered he'd been having an affair with a woman at work...
So I have no idea what you should do. But that's what I did. And 20 months later, I couldn't be happier.
You will have all the support you need here, so keep posting. And you will be ok, whatever you decide to do, so long as you are true and kind to yourself.