if it was their dying wish?
1) you know for 100% sure they are definitely dying and are not just lying to get sex
2) you cannot catch what they are dying of by having sex with them
3) the thing they are dying of does not make them incapable of sex, or disgusting to look at
4) they have between 2 weeks to 3 months to live from the moment they ask but neither of you know how long
5) having sex with them will neither cure them nor make them worse
6) they are a good friend of yours but you don't fancy them