I'd just tell them that we don't think government should be involved in private contracts like that at all except to register the agreement and help deal with disputes. We support the principal but don't support the method of execution of the principal. For example, we support the removal of Sadaam but reject an undeclared foreign war and the stationing American soldiers abroad to do it.
The LP's gay rights credential is the best there is. These other parties are only trying to manipulate the people for their advantage and power at the expense of true freedom for gays. I want all citizens, including gays to experience and express their freedom to the max. AB205 and AB17 don't do that. I'm sympathetic to the practical argument but liberty doesn't need to be sugar coated. It's already delicious.
Just my 2 cents