my gf wants me to fist her anus in return for a blowjob.... should

Sounds lik a good deal!  But you and here need to set ground rules for the anal fiting.  First when she says stop or slow down you need to do just that.  Wait until she isready to start again.  You needa lot of lubricant in order to comfortably insert your fist into her rectum through her anus.  You hae to take it slow and easy, watching her body language: if she tenses up you need to stop and talk her through relaxing.  You may need to start over just to apply more lubricant.  The lubricant I suggest is a personal lubrcant from WalMart or any other store.  Make sure it is water soluable and does not have any anesthetic agents to deaden any discomfort for her.  Discomfort and pain are the best communicators dealing with anal.  Finally, take you time!

thanks bro! just bought this lubricant called ID Backslide anal lube. should work well!:-)

If you do not like it then do not do it, she can not force you.

I would do it but definitely make sure that she knows that a fist is the size of like 5 penises.

no one (including me) likes a surprise like that

have clean her anus before you do it