There is quite a bit of social momentum against the very idea of anal sex for various reasons, most of them religious in nature and at this point they've been taken in as moral obligations by many. One way to poison the thoughts for many is to spread tales of disgust. Think of all the things people did to dispel masturbation habits: hairy palms, bad vision, homosexuality, and many more.
If you do some looking around and talk to some doctors that have experience with gay populations and those who have anal sex more often, you'll find that there are few issues when proper precautions are taken. Even decades and decades of active "use" show no greater signs of issues as compared to the non-anal crowd.
Proper precautions meaning not forcing things, making sure there is great preparation, proper lubrication, and stopping for any pain or damage. There need be NO pain during anal if done properly and with patience. (Far more patience than many possess.) Like any exercised muscles, your body will adapt. You won't be loose, you won't be incontinent, you won't suffer late life indignities. Yet the old rumors still abound.
There is more and more evidence that even those who enjoy fisting frequently have no issues in the future provided it isn't a daily occurrence and again, stretching and lubrication are considered.
Force things? To little lube when needed? Fucking while in pain or with tearing? Yes, you risk future issues.
Other infections are possible but not wildly common. You can be vaccinated for some things like Hep, HPV, etc. UTIs are possible with men who top but not very common for many reasons, not least of which is the length of the urethra.
I've topped in relationships without condoms and had exactly one UTI that can't be traced to that with any reasonable evidence. None of my friends have ever mentioned issues. (We're pretty open as a group.) I know no one personally who has any STI issues in my group of friends, etc. The risks are there; but, they're not something that should stop you from living life with caution.