Prostate health, regular prostate massage, semen retention

This post is about prostate health, and not about psychological or sexual benefits/detriments of prostate play, or the effects on the rest of the body.

When I told my dad about prostate massage, his first question was "is it safe?" Not that he himself has any desire to partake in it, despite his prostate problems (I believe he has had BPH for a couple decades, and he is in his mid 70's). He just wanted to make sure his son isn't going to have serious prostate problems in his later life due to deviant sexual behavior. I had a few vague arguments supporting the safety of prostate massage for sexual pleasure, but they were mainly based on anecdotal evidence gleaned here on this forum. I didn't talk to him about semen retention, as we didn't get into the nitty gritty details of his son's obscure sex practices, but it seems that there is research that links regular ejaculation to prostate health.

For prostate massage:

I am not aware of any large scale studies about prostate massage and its long term benefits or detriments to the prostate. The only non-sexual positive effects seem to be relief from symptoms of certain prostate conditions, and these seem to be supported through anecdotes and through companies selling the devices (the former being biased in the sense that it is likely only those who reap benefits of the masssage post their experience, and the latter being inherently biased). The negative effects seem to be hypothetical for those with prostate cancer (that it might encourage spreading of cancer), and anecdotally I have read that some have done serious damage to their prostate and surrounding tissue from overly aggressive massage (which can easily be avoided if one follows the "hands-free" advice and one is careful not to fall on the stem of their toy).

For semen retention:

It seems there are studies that show links between regular ejaculation and prostate cancer, such as this one:

https://www.ncbi.nlm.nih.gov/pubmed/27033442

This makes me wonder if extended and regular semen retention is really a healthy thing to do at a young age. The above article concludes that those who ejaculate more frequently are less likely to develop certain prostate cancers. It uses monthly ejaculation frequency as a standard, comparing those who ejaculate 4-5 times a month to those who ejaculated around 21 times a month.

Any articles or thoughts that would be good fodder for arguments next time someone asks about health benefits/detriments, or that might better guide me and other users in keeping our prostates healthy?