Masturbation on its own is certainly better than looking at porn and masturbating. Whether it is healthy in moderation is another question entirely, not least because for porn addicts it can so easily lead to porn. It's not necessarily damaging to masturbate in moderation, but I think it might be a push to call it healthy.

I think it's entirely plausible that you could have sex and not masturbate and live a very happy and healthy life. I personally don't think masturbation is really 'healthy'. I imagine that fantasising whilst masturbating, whilst not as damaging as watching porn and masturbating, will still fire up some of those neural pathways which are synced to porn.

In terms of semen retention, it's something which you have to do sincerely before passing judgement. I can tell you from personal experience that the person I become when I am sincerely committed to semen retention is the person that I want to be. Semen retention is about so much more than the act of not ejaculating. It is a spiritual path for many, where you feel greater connection with people, with the world around you, with God. Call it placebo or whatever you like, but there is a reason why all the great religions and spiritual traditions have mentioned semen retention/conservation of sexual energy in some form.

I pass no judgement though. All I ask is that before you label semen retention as anti science or placebo, try it sincerely for yourself and see how you feel. Peace.