If two people wish to actively engage in a homosexual relationship, they're free to do so--with every consequence which comes with that. However, they are not free to redefine marriage to include said relationship. Why? 1. They didn't define marriage to start with, and 2. They don't have the intrinsic authority necessary to redefine marriage. It would not be Christ-like /not/ to tell them this, or to simply let them go on their merry way without warning.

Simple belief isn't necessarily correct. One can certainly /believe/ anything, but a wrong belief shouldn't be allowed to proceed unhindered, simply because the interest of pursuing knowledge is /objective truth/, not convenience. Saying that a particular belief isn't true and/or that following that belief results in grave consequences isn't forcing someone to believe that. It is presenting the objective truth in contrast to a false, subjective belief.