Semen ejaculation could be forced either by electroejaculation without causing an erection or by injection of a chemical that forces an ejaculation with minimal erection. Both procedures are realistic and done in stock animals.
The subject would be out of control and could react in different ways, from excited to nearly panicing from their body building up a chemical induced ejaculation or semen being electrically forced out without any further interaction (or way to prevent it for the subject). This could easily tie into more heavy bdsm/submission desires of a subject.
It would fit a lab environment very well and cater to all ways of speaking to the subject, too. Different lab equipment would be needed to be crafted for this, from chemical substances to electrical probes (including electrical sounding tubes that can be inserted into the penis while the tube won't block semen flow).
While looking through the suggestions I did not find the specific suggestion of flacid ejaculations, so I wanted to add this here.
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