From what I've read over the years (both on Beast Forum and the old Zooville), people have said that having anal sex with a male dog feels basically the same as having sex with a female dog. One difference, though, is that male dogs have a prostate gland (like male humans), meaning that when a human puts his penis into a male dog's anus, it gives the male dog pleasure. You should look at this thread:
One of the pinned topics in the How-To section (before the security breach) was about anal with dogs (i.e. a human putting his penis into a dog's anus). Now that pinned topic is gone. Does anyone have a backup of it, and if they do, can they re-post it? Edit: now that the archive is up, here is...
www.zooville.org
The thread link above contains quotes from 6 different anal-related threads, all of which were on the old Zooville (those threads are now lost, but the text from those threads has been quoted within the above thread link). In the above thread, there is a Natsu44 quote, in which he describes what it was like to anally penetrate a dog for the first time. Also, the following was said:
The threads on the old Zooville said that when a human puts his penis inside a dog's anus, the human should use lots and lots of lubricant (preferably water-based lubricant). Those threads also said that anal sex should occur only with large dog breeds.
Also, you may want to see this thread:
Has anyone had anal sex with their dog, horse, etc.? It can be receiving (such as a dog's penis inside a human's anus), or it can be giving (such as a human penis inside a dog's anus). If so, how often do you do it? There used to be a pinned topic in the "How-to" section about this, but it's...
www.zooville.org
In the above thread link, some people say that anal sex with a dog (human penis in a dog's anus) feels better than vaginal sex with a dog.