Is it illegal to...

Now I have been wondering, is it necessary illegal to have a civil union and a marriage at the same time? Even though the two are similar, they are still not the same and marriage is still the greater counterpart. So is it possible for a gay guy to marry his best friend (female), who wants to marry him and move in with him as friends. But the guy could get a civil union to his lover (male) and it won't necessary count as marriage.

Does this polygamy law mention anything specific about civil unions, or is it just for marriage? Also can a heterosexual couple get a civil union instead of a marriage?

it depends on which nationality you have.