Thanks for query,
You first understand the man's sexual act.
Sex in our mind causes first arousal then erection then ejeculation ended by release of semen.The flow of ejection is involuntary & we can not control it nor regulate it.
never causes impotency.
There is a difference in experience of masturbation & actual sex with girl as in masturbation we have control of pressure on organs.
So nothing to worry about stream or drops.