Christians, why is the brain geared to feel pleasure during ejaculation, when sex is meant for reproduction?

Why during the release of a fluid? Why do we not get the same experience when spitting or urinating?
And if the orgasm is because we have just produced a child why is the feeling not continued throughout the life of that child? In fact it is only usually felt again when ejaculation happens again.
Update: Pleasure being required as motivation is not a valid answer, as Christians also pray, fast and flagellate
Update 2: .
Only answer if you can say why we have pleasure in that and not other fluid releases!
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