Semen spillage - why? How to stop?

My boyfriend and I have a great sex life but when we have sex every time he pulls out after ejaculating all of his semen comes too and makes a mess, followed by a clean up which ruins the mood. It’s never happened to either of us before and it’s really frustrating. Does this happen to anyone else? Does anyone know why? Or how to make it stop (short of using condoms)? We’re both quite fed up and no-one talks about it so I don’t know if we are alone in this!