Of course, it is not about not ejaculating. (If that were the case, tantra wouldn't make any sense for women, for one.) Nor am I convinced that a mystical energy resides in the semen that gets lost.
To me, it is mostly about letting go of any "you HAVE to"s. When you cultivate this inner pleasure that is not dependent on an outer result, after some time the outer result actually does become rather unnecessary, and you gain more independence of outside influences.
So it would be illogical to say, "you HAVE to hold back the ejaculation". Maybe at the beginning, you'll have to do that, because the urge is so strong, and you're so used to associating sexual pleasure with it. But at some point you realize that that's just the prep school. That prescription, that restriction was good for a while, and then it just fell away. You found a pleasure that is so much deeper than the ejaculation, so you will automatically go for that. And if that is not the case (because no prescription fits every human being), then perhaps the wise thing is to just let go of tantra, and do something else!
It is more a practice than an idea or a prescription. Like practicing, I dunno, let's say karate, or rock-climbing. Sure you accept some restrictions, maybe stick to a diet or a harsh training routine - but the restrictions are not the point. You just commit yourself to whatever it is you are doing, and after a time, you discover that there's even more to it than you thought - or perhaps something completely DIFFERENT from what you expected. And then you might stick to it for life. Or, quite possibly, you lose interest, other things become more important, and you just stop practicing... maybe to pick it up again a few years later.
Here's what I've learned from practicing zen: The urge to turn it into a "MUST --- or otherwise...", into an eternally true prescription, into an "only way", a religion, is extremely strong. I certainly fell for it, and I took the decision to stop the meditations at least for a while. And I still stand to that decision, I think it was the right one. Because once you start doing that, you have an internal struggle going on, like a competition - who will win the race, me or me? And that is precisely what it is NOT about. Quite the opposite, really.
But then, sometimes, you "get it", you realize that it's about the doing, not about the result. Those are the times that are really, really good. Those are the times that keep you wondering what else you might discover on this path you've chosen. And those are the times that deepen your commitment.