There's an interesting video series that describes the effects of watching large amounts of porn on a person's psychology that I think might explain some of the article's claims about the dating behavior of Millennials. If you're interested, you can watch it here:
http://yourbrainonporn.com/your-brain-on-porn-series
As far as a significant other's sex with prior partners goes, consider the fact that any given individual's personality is in a constant state of flux. That is, you aren't exactly the same person that you were a week ago, or a month ago, or six months ago, etc. because you're steadily and continually changing as a person with the passage of time.
Similarly, just because my hypothetical partner had sex with someone else in the past doesn't mean that they're somehow less committed to me or anything of that sort. Their motivations and thought patterns have changed since then; that much is obvious, since they'd otherwise still be with their old partner, or at the very least would be too interested in them to be pursuing me.
Because of that, it doesn't bother me when a romantic interest of mine (potential or otherwise) has been sexually active in the past, since that was during a different period in their life and thus isn't particularly relevant to the present. Given how much I change and how relatively quickly said changes tend to occur, how can I look at others and expect that they've somehow remained static?