a) only access the sites when he is not in the room
b) use software to clean all your internet traces (cleandisk decurity is a good one)

And remember to seperately clean google search engine memory cache (if you have a google toolbar and you have used it to search for 'not at all porn but useful BDSM resources')

This way you should be able to avoid him seeing what you are accessing unless he is the sort of computer whiz some of my friends are who can undelete deleted things (but cleandisk mashes up the data good enough to prevent even more of them). Therefore you will not upset him over much.

Alternatively, if you have the cash, buy a laptop (they are actually very cheap these days) with a wireless adapter in it (assuming he has a wireless router, if not you will need a cable to connect it up) and do all your BDSM stuff on that. If it is wireless you can even do it in your bedroom alone where no one can see you (unless you invite them).

Just a couple of practical suggestions to your problem.