Nonoxynol-9 is a horrible substance that's added to lubricated condoms and a lot of sexual lubricants. It is not as effective against HIV as was previously thought possible, and many people are allergic to it (a personal hypothesis is that nonoxynol-9 sensitivity exacerbates and makes more likely a latex allergy).

For many of my friends, nonoxynol-9 hurts when they try to use it. These are also the friends who hurt when they try to use latex. And for myself, I can deal (albeit a bit uncomfortably) with latex condoms, but add in nonoxynol-9 and I can't deal with the burning.