If they don;t have STD to begin with and stay exclusive to each other how are they going get them?
Are you like 19 or something?
Try again Sam. 
Quote #1
"What is Anal Cancer?
Simply put, cancer is an abnormal growth of cells. Anal cancer is abnormal cellular growth in and around the anus. Facts about anal cancer include:
In the general population anal cancer is fairly rare; about one in 100,000 people.
In men who have sex with men, the incidence climbs to about 35 in 100,000.
MSM who are HIV positive are twice as likely to get anal cancer than MSM who are HIV negative.
While anal cancer affects the anus and nearby anatomy, it is actually a type of skin cancer.
Anal cancer can be successfully treated if discovered early.
While anal cancer is not limited to men, it is the only cancer more common in men than women."
Quote #2
"For this reason, anal sex is the riskiest form of sexual activity when it comes to the transmission of HIV/AIDS. Tiny tears in the anal tissue are like giant superhighways for the HIV viruses, allowing them to get inside the body and enter the blood system. Anal tears provide an opening for all the other STDs as well. It may be possible for repetitive anal sex to lead to weakening of the anal sphincter, which is the muscle that tightens after we defecate. Once weakened, feces can escape the anus against our will. However, on Dr. Drew’s website [[http://www.drdrew.com/]], it says that doing Kegel exercises can help strengthen the anal sphincter.. The Kegel exercises are usually used to strengthen the muscles near the vagina. The exercises consist of tightening and relaxing the muscles that you use to stop the flow of urine. These muscles also help stop feces from escaping the anus."