Boyfriend's height 185, me 155. His family doesn't agree with us being together, saying that the height difference is too big to match.

First of all, as your family, they should not object, as long as you are happy. Height is only one-sided and cannot be changed. You can introduce other examples to them and tell them that they are not laughed at now and are very happy. I think they will understand! Finally, I wish you both happiness forever!