This is consistent with my studies in Taoist and Chinese health. For women, intercourse is the best way to absorb and cultivate sexual essence. For a man, unless he comes inside a women; that sexual chi energy and essence(jing) is lost.
It's not quite that simple.
Prenatal jing is essentially the lifeforce that you're born with. It's like a battery that you can't really recharge (unless perhaps by some higher level methods). This is why the ancient masters striving for "immortality" sought to conserve it.
Simply getting sexually aroused will start converting your prenatal jing into postnatal jing. Ejaculating it (anywhere) will release it. But forcefully retaining it at that point will cause jing stagnation (unhealthy). So either way, you've basically lost it after that point. There may be some loopholes to these routes - but only at much more advanced levels of ability.
So for the average layperson, once you get aroused to have sex...you've basically already passed the point of no return then. Holding back after that will only hurt more than help.
The other option is to stay celibate and cultivate your energy until you develop greater abilities to naturally control it. That takes TONS of time & discipline though - and is really only a lifestyle for serious spiritual adepts. After which upon reaching sufficiently advanced levels, perhaps then you could successfully navigate a middle road if you so chose... But again, this is probably not a realistic option for your average joe who is really just looking for some quick fixes.