I believe that what the researchers are saying is that the lectin compound BanLec found in Bananas appears to be a very effective at binding to the HIV virus and thus might be quite useful in a cream or ointment in the, um, chosen area of penetration prior to engaging in sex.
I am not sure what, if any, benefits might be obtained merely by eating bananas, but I rather doubt that they would be significant given that bananas are a staple food of some areas of the globe which have the highest incidence of HIV.