It's probably a pop-under (not a pop-up). And, although you may USE Chrome as your main browser, your phone probably still has the stock Android browser registered as its default app for opening websites.
It's probably an app you've installed that has an ad engine running in the background, or it's something that was triggered from an email attachment, or something similar. There could have even been a time when you clicked on a link in an app somewhere and it opened the site in the stock browser, and the stock browser opened a porn pop-under in a new background tab without you noticing.
This stuff is not unheard of. I think people here are just giving you a hard time because of the entertainment value in a guy getting busted by his wife for looking at porn.
1. Install an antivirus or antimalware app (I'd suggest Kaspersky)
2. Let it scan your phone for ad servers, etc
3. Show her the results of the scan if it finds anything
4. Profit