It doesn't make you a horrible person. It makes you someone who made a mistake.
Whether I would forgive someone depends on the situation. I'm out to my family anyway; obviously Hubby, Alt, and Country all know, as do my parents. I have other relatives, but I don't really know any of them and never have contact with them, so they don't matter as to whether they know I'm poly or not. However, Hubby has asked that his family (parents, siblings, etc.) not find out that I'm poly, so if someone outed me to them, I would be angry--but not as angry as Hubby.
Does your partner know you outed them? If so, what did they say about it?