 as for semen even if its release is natural it is mostly dependent on a voluntary act which is doing something that requires it such as intercourse and the like.
وأما المني، فإن خروجه وإن كان طبيعيا فهو موقوف على الأكثر على أمر إرادي وهو فعل ما يوجبه كالجماع ونحوه.