I don't agree... I'm pretty sure you can tell beforehand that you'd suck at something without having tried it, judging from what you know about your likes, dislikes, general personality, talents and incompetences.
Also, being a polyamorist myself (basing my polyamorist conviction in my religious beliefs as a non-confessional pantheist mystic, btw), I don't like the concept of "one destined person somewhere out there" at all either. If someone told me that loving him/her meant I shouldn't love anyone else at the same time, they'd be in trouble - it would never work out, they'd probably never feel loved by me, as there will always be others. Exclusivity just isn't how love works (at least, the way I understand the concept 🙂 ).
So, definite no to marriage or any other exclusive commitment type relationships, from my side.