IV drugs ==> A way to spread AIDS
Anal sex ==> A risky behavior for spreading AIDS
IV Drugs and Anal Sex ==> Enjoy your AIDS!
But being a little more serious, anal sex helps to clear out the rectum of feces (makes it come out). So maybe the anal sex actually helps them out, gets them at least temporarily un-constipated, so they can go back, do more heroin, and get constipated again, then go back and have more anal sex, shit, do more heroin... It's an endless cycle.