The Catholic pope recently endorsed same-sex marriage. The Catholics are not the first so-called Christian sect to do this, and they won't be the last.
The issue here is not same-sex marriage. It is the endorsement of sin and the allowance for people to remain in the sect's church while in sin, which is contrary to the Lord's commandments.
The idea that you can willfully sin and still remain in a Christian church is a relatively new one. In Mosiah 26 we read that it was the first time that people openly disobeyed the commandments and still thought they could remain in the church Alma had founded. It was so plainly understood that you have to actually repent to be baptized into the church that they had never had to deal with it before.
Alma inquired of the Lord, and the Lord replied:
27 And then I will confess unto them that I never knew them; and they shall depart into everlasting fire prepared for the devil and his angels.
28 Therefore I say unto you, that he that will not hear my voice, the same shall ye not receive into my church, for him I will not receive at the last day.
29 Therefore I say unto you, Go; and whosoever transgresseth against me, him shall ye judge according to the sins which he has committed; and if he confess his sins before thee and me, and repenteth in the sincerity of his heart, him shall ye forgive, and I will forgive him also.
30 Yea, and as often as my people repent will I forgive them their trespasses against me.
32 Now I say unto you, Go; and whosoever will not repent of his sins the same shall not be numbered among my people; and this shall be observed from this time forward. (Mosiah 26)
Ezekiel quoted the Lord as saying:
Her priests have violated my law, and have profaned mine holy things: they have put no difference between the holy and profane, neither have they shewed difference between the unclean and the clean, and have hid their eyes from my sabbaths, and I am profaned among them. (Ezekiel 22:26)
This is what is happening today.
There is not one organized religion that actually keeps God's commandments as far as they know them. Not one.