No, this is not safe. There is bacteria around the anus which can make you sick.
Frankly, I think all this degrades you. What's next?
First let me say you are one awesome girlfriend if you let him do this kind of stuff! But I gotta say, you probably need to dump this guy before he ( literally ) dumps on you. Im sure thats what he is thinking about while he is getting off, and its only a matter of time until he ( literally ) lets loose. Thats not something you are going to forget, so you better get out now.
show the guy the door and don't look back. This is pretty sick, degrading and potentially dangerous to your health.
Jim
this is not for real people!!!!