what about when his future girlfriend or whomever asks "do you have any diseases?" and he trusts his past girlfriends to fill him in on that minor detail so he says "no, no one has ever contacted me telling me that i should be tested for anything." so they dont use a condom.

im sorry, but if you cant even be honest with your boyfriend of a year and a half then maybe you shouldnt be together anyways. it has everything to do with people in the future. thats horrible to suggest that if they contract something when he has no clue that its their own fault. id like to see how calm and collected you would be if that happened to you. im sure you wouldnt calmly sit down and say "well, i can see why she wouldnt tell him. and i trusted him enough not to use a condom so this cancer i could get is totally my fault." no, im sorry. i can almost guarantee that nobody would react that way.