You are in a “damned if you do and damned if you don’t situation.” Let’s try to simplify it the best that we can.
You have strongly held biblical beliefs that your daughter is wrong. As such, you want to communicate your beliefs to your family (and daughter) so that you won’t be betraying God. I completely understand this. You do not want to endorse the marriage but you want to continue to have a relationship with your daughter because you love her. You are a great mother. Some would call you unaccepting, narrow-minded, homophobic or bigoted, but this is wrong because you are completely entitled to hold fast to your beliefs. If someone attacks you for disagreeing with them, they are bullying you. The only thing that bullies can resort to is calling names so ignore them. This is hard, particularly when love is involved.
I have been involved in different ways with similar situations both professionally and personally and here’s how I have seen this reconciled. Your job as a Christian is to do two things: love God and love her. It’s that simple. You know that you love God and now your job is to simply love your daughter. A good friend of mine, in her thirties, felt as you do when her sister was marrying another woman. My friend went to her sister and said, “Here’s the deal. I love you like crazy. You are my sister and I will always love you. I completely disagree with your lifestyle and you know that. So, from now on, we don’t need to talk about it anymore. You know how I feel and I’m entitled to my feelings and beliefs. I respect you and realize that you have a right to your beliefs, too. I will always be here for you to love and help you.”
As a Christian, your primary goal is to help her grow closer to God. The best way to do that is to stay by her side and love her. This doesn’t mean that you have to change her, argue with her or condone anything that she is doing. You just have to love her along the way.
As a mother your goal is to teach her (you have communicated your beliefs) and if she disagrees with you, that is her right too. Sometimes we get caught up trying to change someone’s behaviors before we help them love God and I don’t think that’s the order that God desires. First, our goal is to help them grow close to Him. Christopher Yuan is a brilliant man, lived a gay lifestyle and now teaches at Moody Institute. I asked him about attending wedding ceremonies of gay friends and he said that he would. Why? He said that he didn’t want to cut communication with loved ones and since they knew his feelings about the marriage, he wasn’t worried that his presence showed that he endorsed it.
Your daughter who says that you are splitting the family is wrong because it sounds as though the family is already fractured. The remaining question for you is: how will you love a daughter who exhibits behaviors and a lifestyle that you find offensive? Many of us do this all of the time. Women are married to men who offend them and men are married to women who do things that they wholly disagree with. Trying to love someone in a messy, confusing world is nothing new. There is only one-way to do it well, keep your eyes on God and ask Him for help. Your job is to love her; His job is to reconcile her lifestyle choices with her and her alone.