I do not see the problem being some kind of slippery slope that will open the doors to all those other kinds of arrangements you noted. The problem I see is legal marriage between two men or two women.

Condoning Gay marriage will in fact increase homosexuality in a culture. The great majority of practicing homosexuals were nurtured in an accepting environment, not born with some gene. So the more nurturing the more prevalent will be the numbers. Now I suppose if you have no moral objections and no misconceptions that there are spiritual ramifications for sexual activity outside of marriage then there is no strong case against it. But if you do believe that some God above does not approve (reference his Word for example) then perhaps a segment of society has legitimate concerns? I do not think God is very happy with the moral condition of the USA or Western Europe, former bedrocks of Christianity and moral consciousness. It appears we are spiraling down to a predetermined tragedy that will unfold either slowly (as it already has) or more suddenly on the national landscape. You have been warned.