Why does my male cat kep pinning down and biting the neck of my female cat?

Unlike most animals, Female cats don't really choose thir mate- they just fight him off or don't - The male is mating age and is ready to go. He's trying to pin her down to have impregnate her. Get him fixed if you can only do one right now, he will begin spraying your house and he may have already impregnated the female- tho she may be to young yet. Eventually, get her spayed.