If they do, it had better be for the right reasons.
Christ is a stumbling block. (1 Cor. 1:23) And Christ is a corner stone. (Luke 20:17) He is a big, heavy rock that can either be tripped over, or upon which the most durable structure can be built. Jesus is Truth. His teaching, His statements, and His reality are hard to believe and even harder to act upon. Jesus said that while He greatly values unity, sometimes the truth of who He is and what He says will divide. (Luke 12:51) It’s not a prescription; it’s simply a statement of fact.
Here’s the challenge for His followers. Don’t add any extra stumbling blocks for those around you. Neither your tone, nor preferences, methods of communication, human tradition, your sinful motives, your selfish desires, or even your freedom should impact unity. This is called dying to self and we’re supposed to do it daily. Strive for blamelessness and holiness which both Christ and Paul indicate are realistic goals for believers. (Matt 5:48, Phil 2:15, 1 Peter 1:15-16)
Christ and His truth should be the sole factor that divides. So what does this mean for our discussion with non-believers about gay marriage? (Or for interactions with anyone outside of the Body of Christ.) Do not withhold yourself and thereby cause them to stumble. Do not allow lack of tact or sensitivity to cause them to step away from you or Christ. Do not let them trip over words of truth that are not followed and preceded by actions of sacrificial love. Hypocrisy, wrong motives, secrecy, impatience, selfishness, misuse of scripture, lack of repentance- all of these can drive a wedge between us and others and damage their view of Christ. We must not elevate any issue above the Person of Jesus. If someone stumbles, it had better be Christ they are tripping over, not you.
Jesus said that no one comes to the Father except through salvation in Christ evidenced by costly action. (John 14:6, Luke 8:21) That is enough of a stumbling block. We sin when we add our own tripping hazards to the conversation.