I'm going to try and summarize some of the discussions that I've seen and participated in here, along with the policy itself.
First same sex marriage is grounds for a disciplinary hearing.
Second children of same sex couples must be legally adults and disavow the practice of same-gender cohabitation and marriage.
Children are being punished for the sins of their parents
The church is driving families apart
Same sex marriages are no different from any other family arrangement out there, since we're all sinners.
If the church is requiring children of SS couples to wait till age 18, why not require it of all children?
But what about [this one type of circumstance that might come up] where it doesn't seem fair?
Children are being punished.
No, they're not. If the bar for punishment is being denied the Holy Ghost for any period of time, then we're all punished before age 8, and all people born between year 90 and year 1830 or so are also being punished and all black people were punished until 1978, etc.
Remember that God operates on an eternal timeline. Nobody, nowhere, no time, is ever denied baptism and the gift of the Holy Ghost. For the vast majority of the world it will come only after death. None of those people are being punished. God could have given a new dispensation with the ordinances of salvation at any time he wanted. We do not accuse him of having punished those who did not receive them because the miracle of the restored gospel helps us know better.
God is at the helm. No one has been punished or denied exaltation to any degree.
The church is driving families apart.
Since the rule about baptism requires that children not join a religion which requires the child to disavow the sexual practices of his or her parents, this argument holds no water. If anything, God is relieving such children of the requirement for baptism at age 8 and granting them 18 solid years with their parents where they wont have to endlessly wonder if they're doing the right thing or not. For many this will be a remarkable psychological relief.
If the church is requiring children of SS couples to wait till age 18, why not require it of all children?
This argument asserts that there is no substantive difference between a child from a SS marriage or one from a traditional marriage.
I think it's self evident that there's a wide spectrum of family types and children. Thus the idea that all children not from SS marriages are the same as those that are is false. Consequently, this means that the reasons behind this policy can't have anything to do with the nature of the children themselves.
Same sex marriages are no different from any other family arrangement out there, since we're all sinners.
This is most likely where the hang-up is. I think that we interwebosphere people for some reason downplay just how serious a problem same sex marriage is on an eternal perspective. Barring some unforeseen revelation about the nature of exaltation or marriage, we know that those in same sex marriages can never achieve exaltation.
Other than the denying of the holy ghost, there is literally only one other sin that carries that same level of consequence with it: Murder. It absolutely bars the way into the celestial kingdom.
It may be that we are all severely underestimating the seriousness of the consequences of same sex marriage.
If the church is requiring children of SS couples to wait till age 18, why not require it of all children?