Unfortunately, the modern world has made this matter even more complex. I might have thought a few years ago that marriage should remain a wholly heterosexual institution, but there’s also considerable legal benefits to marriage that a gay couple forbidden to wed wouldn’t have, such as social security, inheritance, health insurance, medical decisions, and marital tax deduction, just to name a few.
Last time I checked, the “modern world” does not even believe in a life after death, so making any decision based on what the “modern world” believes is bereft of any truth - or TRUTH (aka Jesus Christ).
Tax law, as has been pointed out, is a construct of the Federal and State governments and is subject to change. Allowances for a subset dealing with homosexual couples could have been made, without the necessity of calling such relationships marriage.
Social Security is a separate issue; they would not have access to some provisions made for married couples. However, they still would have their own SS benefits as if they were single; non-marriage does not take that away anymore than it does to any other single person.
Inheritance is also a matter of State and Federal tax law - see above.
Health insurance is also law, created by lawgivers and subject to change - as we have seen in the last decade. HIPAA laws likewise could be changed to include partnerships. And medical decisions likewise. The marital tax law, see above.
And none of these issues are ones subject to grounds for overturning marriage as a man and a woman - which goes far far beyond simply Judeo-Christian law; it is near universal.
Not to mention that none of what you have listed has to do with the moral law and their individual souls.
I do not suggest it is easy; but it is simple: you can either speak the truth, in charity, or not speak the truth - which in my book, bespeaks a lack of charity.
Priesthood is a sacrament; and as a sacrament and as an alter Christus, it is up to you to proclaim the truth, and the TRUTH. No one ever said that is easy; and no one ever said that the listeners were of necessity eager to hear it and him. It is up to you to speak the truth which the Church holds, and then pray mightily to the Holy Spirit that the Spirit will lead them to that truth, no matter how hard they perceive it, no matter what the world says, and no matter what their emotions deliver to them.
You are not responsible for their decisions; but you are responsible for proclaiming the truth; it is up to them to accept or reject it.