I have been in and closely connected with ESL programs in the USA as well as EFL programs--to a much less extent. From that experience, I would answer that there is no 'conspiracy' by the university systems that would be of the 'dating source' or 'hook-up' type.
It is true, however, that the age, the circumstances, and all the little workings of nature are there to make relationships happen, The ESL programs are usually slightly detached, in a way, from the main school, and thus the area of movement and interaction between students in the programs is usually only between and among students of that same program.
This could be said to be fertile soil for whatever relationships that develop--which would have likely developed anyway, anywhere due to those above points--to develope only within those participating in the program.
I got into a relationship with a Japanese girl in an ESL program. Another Japanese girl there was in a relationship with a fellow student from Korea, and yet one more Japanese girl ended up marrying a university student who she met through the ESL's 'open house' culture sharing 'festival,' and a few Japanese girls were in relationships with Japanese boys. Among some 35 or so Japanese students in the program, it was not above average for the age, I'd think.