if the smell is completely out of the ordinary, you should also consider that it may not be the sperm that smells. so i would say that both of you should go get checked out.
Hello,
Well it's normal for the semen to leak back out if it's unprotected sex, but usually it doesn't have much of an odor to it, if any. What type of an odor is it? It could indicate one of you having some type of infection if it's particularly foul smelling and accompanied with any chunky type discharge. For example, it is possible for men to get yeast infections (which I never knew until a few years ago) and to pass it back and forth between the male and female. I'd definitely get it checked out if you think the smell is out of the ordinary and definitely use protection until you do. Good luck :)