In the book "Foreign Sperm" by Donald Tyler, some interesting questions are raised regarding the potential of sperm. The heart of Tyler's theory is that foreign sperm, any sperm into a woman, and another male's sperm into a man, cause diseases.
If Tyler is correct on any point, the absence of ejaculation during karezza may be a huge benefit to long term health.
A book description, contents, introduction, and conclusion of "Foreign Sperm", under the old title, "The Other Guy's Sperm" is available here --- http://www.davidpratt.info/tyler.htm
A few copies of "Foreign Sperm" for sale can be found here --- https://www.amazon.com/Foreign-Sperm-Donald-E-Tyler/dp/1884981097/ref=sr...
The photo of the back cover (on above link) gives Tyler's background.