You should be able to marry with whomever you choose, or however many you choose (within the confines of your religion). However there needs to be a separation between marriage and legal status. Marriage is a religious thing, period. It shouldn't mean anything, grant any status or rights, or involve the government in any way. Unions should be the government version of marriage (which can optionally be done concurrent with a marriage, or separate from it). A union is what would grant status and rights (tax things, inheritance, etc). I think a union (of marriage) should be restricted to one person, purely for logistical reasons. There might be need of another type of union for offspring (this would settle issues of having children with multiple permanent partners).
So, marry your gay lover and 45 members of the opposite sex (if allowed by your religion)...but you only get a government sanctioned union with one person. This pretty much solves all issues about gay marriage, polygamy, etc.