Well, in my experience most people who want to debate from that standpoint are using it as a crutch to prop up their crumbling belief systems.
That said, from a purely statistical, mathematical standpoint you can't argue the point. In theory - anything but one virgin tagging up with 1 other virgin technically raises the risk. But it's (as Rubyfish) basically said) really a pointless argument.
Life has risks. We take them. We drive over the speed limit. We rock climb. We walk in the city at night without a handgun or mace.
We survive because we are calculated about the risks we take. But we never eliminate them.
Most people who believe in and live a poly lifestyle are probably more aware of such potentials and are therefore proceed with more caution. BECAUSE we are aware, it's a topic we don't shy from and generally bring out on the table right at the first entry into a potential relationship. We ASSUME our potential partner has/had a few - or many, lovers and has a sexual history that may include exposure to STDs. We talk about it openly. It's not taboo - but critical.
How many mono hookups / potential relationships does that take place in ? Not many I would guess. On a first or second date how often do you come out with - "so - how many sex partners have you had and how much do you know about their history?" <chuckle>
In the poly - even swinging, world this is pretty much the opening conversation
(if it's apparent this is headed in a sexual direction)
So you decide which carries the higher REAL risk.