My wife complains about a burning sensation inside her anus during anal sex, what is causing this? .

I don't have an answer, but I'd be very interested to see if anyone else does.  I am a woman who has gone through this same thing...my husband using a finger in my anus didn't bother me at all, but him inserting his penis was very painful.  If nothing else, you can tell your wife that she isn't alone.