Here in California, both civil unions/partnerships and marriages are available to both heterosexual and homosexual couples. For the most part in California, the only difference is in the name. I understand they are supposed to be legally equivalent.
In different places, your results may vary.

For those wishing to split hairs, here is a handy guide: http://family.findlaw.com/domestic-partnerships/civil-unions-v-marriage.html

Ake, where are you going with this, that you haven't gone thirty-seven thousand times before? You're boring us.

-Joe-