i'm going to be a bit radical and say you don't even HAVE to tell the person you're having sex with you have the virus. maybe you should. maybe you shouldn't. it's really you're call.
do unto to others. blah. blah. blah.

if you're asked, that's another story. and "forgetting" is not an excuse.

i have friends who have hsv(1) from loving family members who kissed them as babies. they've kissed countless people. no disclosure anywhere there. why should sex be escalated to a higher level? that just reinforces the "dirtyness" of teh sex.