In the US, civil union is not legally equivelent to marriage. (this message is US-centric)
Heterosexual marriage is recognized in all states and territories, and in all foreign nations. Civil unions are recognized by less than 1/2 of the US, and rarely by foreign nations.
That matters for things like being able to decide where your spouse will be burried, if your spouse dies somewhere other than a place that recognizes same sex civil unions. There's also federal law - civil unions are not recognized as equivilent to marriage by the feds. This isn't just about civil rights you can assign with contracts, either, but also rights and obligations. No contract can allow a same sex couple to claim joint taxes federally (an example of a right most of the time). No contract can apply rules about conflict of interest in federal matters to a same sex spouse or civil union partner (an example of an obligation - I couldn't sign a contract as a federal employee if my wife was the CEO of the corporation getting the contract - but a gay couple could do exactly that.
How does it affect trans people and their spouses? If I was married to a post-op MTF, and I'm a cis-male, and we went to Texas, where she died, her body would be released to her parents, not me as her husband (since Texas, at least some of it, uses the idea of expected DNA to determine sex). Her parents could then put whatever name they wanted on her tombstone. It's even worse for pre-op or no-op people. For that reason (because there is no good definition of legal sex), even heterosexual trans people and their SOs should support gay marriage - to eliminate one of the very few reasons that states have to come up with bogus definitions of sex and gender.
We've made marriage about love years ago, instead of inheritance and property rights. It's about time we finished that change.