The things that get discussed here! Makes me feel like I'm at work..
(well, I am actually...
)
HIV is a very fragile virus, actually. Nearly any physical process that will destroy the protein cells it lives in will "kill" it. Detergent will "kill" HIV on a surface by removing the proteinaceous matter it lives in.
That said, the cells in which HIV lives can be taken into the body EXTREMELY rapidly in certain circumstances. Such as unprotected sex where there are abraded membranes involved. No amount of "after thought" can help in that situation, except for appropriate systemic anti-viral therapy.
And using lemon juice before unprotected intercourse just may cause abrading of the membranes.
So I doubt this will ever be recommended, IMO.
Cheers
Mick