In this paper, I answer the following question: suppose that two individuals, C and D, have been in a long-term committed relationship, and D now has dementia, while C is competent; if D agrees to have sex with C, is it permissible for C to have sex with D? Ultimately, I defend the view that, under certain conditions, D can give valid consent to sex with C, rendering sex between them permissible. Specifically, I argue that there is compelling reason to endorse the Prior Consent Thesis, which states the following: D, when competent, can give valid prior consent to sex with her competent partner (C) that will take place after she has dementia, assuming that D is the same person as she was when she gave prior consent, meaning that, if D, when competent, gave prior consent to sex with C, then C may permissibly have sex with D. In Section 2, I explain both the background and the existing literature on this issue. In Section 3, I outline relevant stipulations about the kinds of cases I will be examining. In Section 4, I defend the Prior Consent Thesis. And, in Section 5, I address objections to the Prior Consent Thesis.