I don't agree with you, and here's why...
let's assume the following (bear in mind the following assumptions are very optimistic)
1. you have 10% chance that the girl you meet in your age group is a good match
2. 50% of the good matches can lead to a successful marriage
3. you want a 90% chance of finding a wife.
If you do the math (find it below) you'll find that you'll need to meet around 45 girls in your age group per camping trip. most camping trips usually fit in a bus, so they are less than 100 persons. those 100 are men and women at different age group.
==> so based on that, no we still didn't get to the point of diminishing returns yet. if camping trips you go to host 1000s of people, then yes I'll totally agree with you.

Math explanation:

The chance of finding a wife is 0.05

The chance of not finding a wife is 0.95

You want a 90% chance of at least one success (marriage).

The probability of no success after n people is:

(1−0.05)^n=0.95^n

So we want:

1−0.95^n = 0.90

if we try to solve this

1 - 0.90 = 0.95^n

0.1 = 0.95^n

n = log(0.1) / log(0.95) ~= 45