An atheist perspective does not have God in it...and that makes all the difference my friend. If God is all powerful then He can make His creation pregnant without insemination.
From a Christian perspective, the reason for the immaculate conception is this, Adam, a man, fell from grace and a man was needed to restore what a man lost. However every man born of a man has the nature of Adam and is this a sinner like Adam. Christ on the other hand, being born of a woman was born without Adamic input so could have a nature different from Adam, a Holy nature, God's nature.
It is this nature that qualifies Him to be able to die for mankind's sin. If a man born of Adam died for sin, he would at best be paying for his sin but if Christ, having no sin nature and no connection with Adam dies for sin He would be paying for sin on behalf of sinful man.
The gospel is based on historical facts, Key of which the man Jesus Christ who lived a life prophesied in the Old Testament (His lineage, city of birth, suffering death and eventual resurrection) centuries before He was born. All of this were recorded by eye witnesses.
So it's not dogma with no proof my friend.
Finally, others can think what they want, no issues with that, but we cannot lie that we agree with them when we don't or have never even considered what they think about things.