How do I tactfully tell my neighbours that I can hear them having sex?

So I live in an apartment and, although noise isn't usually a problem, I can hear my neighbours having sex. LOUD sex. And I can't imagine telling them "hey, could you tone it down while you're fucking?" would go down well. Even more awkward is that one of them is my partner's cousin, so we see each other relatively often. Is there any respectful way of asking them to maybe not scream as much during the act?