There's no polite way to explain it. It's a parasite. A demon. It's hideousness astounds women and takes their breath away. While stunned they are easy prey. The incubus attaches itself.
She never sees it coming because she's asleep. It could be a handsome prince or someone she knows for all she knows. So rough, though.
Either way, it is not a pleasant union. Bile arises. The revulsion requires absolution, but no amount of cleansing will do. And nine months later... You just don't want to know.