If the goal is to have certainty with respect to paternity, then lifetime monogamy is one possible way to achieve the goal, but clearly no longer the only way to achieve the goal. I do not see how two intelligent people with access to very effective modern birth control would have trouble achieving certainty of paternity of what - 2 children per lifetime? 3 children per lifetime? And that would be difficult?

It is like wi-fi - once wi-fi is available, there is no longer a need for a network cable and you can use the laptop on the bed or wherever you want. You are still free to use a network cable if you cannot manage configuring wi-fi, or you may prefer to use a network cable because hard-wired connections might be faster or more robust, but you do not have to use a network cable once the technology advanced to wi-fi.

Likewise, what used to be the only solution to the problem of certainty with respect to paternity is no longer the only solution thanks to technological progress, so while lifetime monogamy does remain one of the solutions to the issue, just as hard-wired connections do remain one of the ways to connect to the internet, both no longer represent the ONLY way to solve the respective problems.