It as absolutely NOT SAFE to have sex when the tip of the IUD has protruded through the cervix. That happened to me twice, two different types of IUD's and the uterus is trying to expell it - you can, and will get preganant if you are in a fertile time of your cycle. The only thing to do is have the OB/GYN remove it and inplant another one - once it starts to come out, it will be damaged and not effective.
sounds like u need to get it checked out. u could try using a condom i suppose as long as it not painful