Why my ex and his new gf make fun of me?

Maybe my question is pretty obvious. But can't people just act civil these days? I dated this ex bf long time ago, but he is still bitter towards me. His new gf also would say in public that they live together so that I can hear. She also says that they would get married once they want to have children. Whenever I write e-mails to him (job related) he responds subtly making fun of me. Unfortunately, they both go to meetings with other common friends, and they don't lose the opportunity to make me feel bad. What should I do? I don't think I deserve it, I don't interfere in their business.