Not sure what you mean by "sponsor." If it's something that involves government licenses or handouts, then you're trying to force people who disapprove of your lifestyle to fund and/or participate in it in some way, which is wrong.
Point is, gays and Christian groups are freaked out about the marriage issue because government is forcing an either/or choice on them. In a natural market setting, the groups would simply not be very concerned with what the other did, and would probably just choose to stay apart from each other except for when they found peaceful economic exchanges advantageous.
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