I think you missed the point I offered. If sex were a driving factor, there are far easier ways to get sex that don't involve the work of a relationship. Sex only becomes a factor within a relationship. Yes, we want the wonderful sex that comes with a wonderful relationship. That, however, doesn't make sex a driving factor to be poly, just an expected outgrowth of a romantic relationship.
And the fact that there are poly folks who engage in deep relationships without sex shows that sex isn't necessary to a deep romantic relationship. That highlights that sex is not a driving factor, necessarily, for becoming poly.
That's also not to say that some folks view it as such. The fact that some do doesn't mean it is a driving factor in general. Some future study may show that such folk are in the majority in the poly community, certainly, which would make support of the notion that sex is a driving factor fully supportable. My experience suggests otherwise.
So, there's no delusion. And, if you read the whole of what I said, you'll find that I don't diminish the importance of sex in any fashion as part of the relationships. I just don't see it as a driving factor any more than I see it as a driving factor in a deep romantic mono relationship. I didn't marry my wife just for sex, nor was it a major consideration--I could get sex without all the work of a relationship and being involved in a relationship has never been necessary to get sex.
Indeed, if sex were a driving force, I suspect one would swing in preference to opening up as poly. I offer that the driving force thus isn't sex, it's the ties of relationship that drives us poly, and sex is just one of many benefits.