I think the frisson caused by the inference that anal intercourse leads to HIV infection captured the imagination of the media at the time, and to an extent still does.
The notion that anal intervourse is a social tabboo links in perfectly with the perception that indulgence leaves people open to a disease-borne retribution is a perfect combination for 'sin' and 'damnation' which have been a cornerstone for moralising individuals since biblical times.
The proven medical facts that support the notion that HIV is now spread more frequently through unprotected heterosexual intercourse is not going to allow a deviation (pun intended!) in the course of occupation of the moral high ground.
Which is easier for the moralists to accept - HIV is a risk for all unprotected heterosexual intercourse with individuals whose sexual history you are not privvy to - or 'Them dirty queers get what they deserve - a lingering painful death' - it's not a contest really, is it?