There isn't an number you can put on it. Eroticism is as mental as it is physical. But the mental depends on the physical, and what is erotic is actual human contact. There is an energy that passes between two people through touch. A condom puts a barrier between you and is like the insulation on the outside of an electrical wire preventing that energy from connecting. Sure, I can feel pressure and friction and whatnot through it, but I can't feel the actual connection you get from touching skin on skin, which that actual skin-to-skin touch is what we are primally driven towards, is hard-coded into our DNA as a biological imperative. Have you ever watched two guys wearing gloves come up to each other and shake hands? They almost always take over their glove to do it. That's because we as human beings need actual human contact that is skin-to-skin. So, when it comes to sex, which is the most intimate expression of that need, wearing a glove and putting a barrier between, it changes the experience entirely and makes it fall short of fully fulfilling the need we're trying to meet with the experience, needs we often choose to leave unmet because of other needs, like the need to preserve our health and the need to remain free of a child we haven't the wherewithal to have at this point in our lives.