That feels like a wild assumption to me - we really think people 45+ aren't having casual sex? less casual sex maybe, but I would imagine still a decent amount, statistically.
And over 45s that engage in casual sex have almost certainly been exposed. Those who don't have prior exposure probably have few or one partner in a marriage or marriage-like situation and thus are not likely to be exposed. Yes, they could be cheated on or the like but exposure is not expected so effectiveness can not be measured.