In same-sex marriages, which partner takes the last name? How is this determined?
My very dear friend and his husband hyphenated theirs. They were able to marry before California got all stupid. 
None of my other gay friends are allowed to be married, so they are the only couple I can comment on.
In Ontario, the arrangement of names is left up to the couple concerned.
The law says that the married couple can have the family name of Spouse A, that of Spouse B, that of A followed by B, or that of B followed by A.
In some states, even for heterosexual couples, both partners get a free opportunity to change their names to whatever they want. Most often, the husband chooses to keep his name and the wife chooses to take his, but it could go the other way around, or both choosing a completely new name. Presumably, if any such states allow gay marriage, the same rule would apply, and it’d be up to the couple to decide.
In the U.S., people generally have the freedom to name themselves whatever they want regardless of most anything with or without legal approval (with a few exceptions). A marriage allows a free legal name change for the bride at least with the purchase of a marriage license which it isn’t technically necessary but often the easiest way logistically. The groom may also get one too for whatever the couple decides on. My ex-wife used my last name personally, her maiden name for business, and the hyphenated version for other purposes without a problem and that is common. Gay marriages have the same rights. If there was a real issue, it just takes a simple court rubber-stamp before a judge to make it legal in government records and databases to make the last name whatever the couple wants. Some people pick a name completely different from anything their families have.
In California, there’s no automatic name change for any marriages, so the two people have to decide for themselves.
FWIW, when we got married last June, we each kept our own names.
And just for fun: In Québec, you keep your own name. You are free to call yourself anything you like socially, but on all legal/government documents etc, you retain your name. Marriage does not allow for a free name change like it does in most other jurisdictions (that I know if, in Canada and the US), and it’s been this way in la belle province since the early 1970s. To change your name you’d have to go through the same process any other random person would have to do to change theirs.
Which worked out well for me, because that meant I never had to decide if I wanted my husband’s distinctive name or not…the choice was made for me! Nope!
Yeah, database ate my post. Short answer: We kept our names.
We’re going to each keep our own names.
Edit: or they can each retain their previous name. I believe that marriage allows the spouses a ‘free’ name change, as opposed to any other time, where you have to go through a more involved procedure and possibly pay more. Link. The linked website (from the Province of Ontario) speaks of ‘assuming’ a name upon marriage, which apparently does not remove the original name.
My daughter and DIL hyphenated their names. They flipped a coin to see whose birth name when first. 
Daughter’s won.
In Spain it isn’t customary to take your spouse’s name. As for which lastname goes first for any common children, the couple decides it. Traditionally in Spain the father’s lastname went first and in Portugal the mother’s, but in both countries it is possible to choose the other order if the parents want. For a same-sex couple, there simply isn’t a default option about which goes first.
Since people are likely to have heard of this custom: in Hispanic countries the wife can add her husband’s lastnames to hers, but this is for social purposes only, it doesn’t go on the paperwork: a Carmen García who becomes “Carmen García, wife of Pérez” would still be Carmen García for purposes of SS, ID, etc. It’s really not any different from becoming “Child#1’s Daddy” when your firstborn starts school; people from my hometown are extremely unlikely to recognize my full name, but if you say “the Navabro’s sister” they will go “oh, yeah, Nava!” and give you a biographical extract.
My good friends (hetero) in NYC have been married over 20 years and they both kept their original last names. They both had extensive business ties and didn’t want to go through the hassle of having to contact everyone they knew and changing anything.
I would imagine a lot of Gay couples would do the same - just keep their names.
For those of you who are married, but have kept your own names- what name have/will your kids take(n)?
Never mind.
No kids. If we had kids, I presume we’d either choose a name in common or, since our state doesn’t recognize our marriage per se, the name of the non-bio parent to help establish evidence for the parental relationship.
In Florida, at least, that rubber stamp will cost you $200 - $300 per person. A free name change for one partner is one of those benefits associated with legal marriage that gays and lesbians don’t have access to.
In the gay couples I know, the partners either kept their original last names or hyphenated their names. No one simply changed his/her last name to his/her partner’s.
We are married in Canada, each keeping our own names. However, we are technically unmarried in Missouri, where we currently live.
When we had our son, I carried/birthed him but we gave him the last name of my partner. This is because there was a 6 month waiting period before we could go to court for a second parent adoption, which would make my partner a legal parent along with me. Just in case anything happened to me (i.e. I died or became incapacitated) in that 6 month period, we wanted to be sure that we established in no uncertain terms that my partner was equally the parent of our child, although I gave birth to him. So we gave him her father’s first name as well as her last name to establish intent, of some sort. (Besides which, we liked those names, which was also a consideration!) Living in something of a backwater like Missouri, you are at the mercy of the personal biases of judges when ruling on family matters involving gay or lesbian parents. Therefore, we did everything in our means, legally and symbolically, to make sure that my partner had connections to our son that even a blind bigot could recognize. Not that it would prevent a bigoted judge from ruling however he or she pleased, but it would offer a more moderate judge some clear guidelines on our intent for my partner to be a parent to our son.
Probably his name; mine’s less interesting. We might give them both, but not hyphenated…mine as a middle name or something.