Lore-wise, would anyone be dissuaded from marrying a human/mer as a mer/human? I mean, with all the lifespan differences, meaning that an Altmer is more likely to outlive his/her, say, Redguard wife/husband and all that...

It does seem to happen, e.g.,

It may be uncommon for the reasons you state, but it doesn't seem to carry significant social censure or taboo.

Lore wise it may not be the "ideal", but other than that "the heart want's...what the heart wants". and their are a couple in game as was pointed out... and I'm sure probably a couple in Oblivion also.

My Character Zyne married Marcurio... and I found a good reasoning for it. She was much older... but it worked for me in the story.

http://www.uesp.net/wiki/Lore:Breton

Marriages between men and mer are relatively common. They are presumably far more common than the games portray.

And in case you're wondering about children, remember that the race of the child is always the same as the mother's.

Buffy's a Bosmer, but her father was a Breton. Buffy fell in love with an Imperial and looked forward to their life together - even though she knew his lifespan was but a few mere decades compared to hers. It turned out not to matter though, for he was killed long before his first gray hair.

All things being equal, she would prefer an elf only because of compatible lifespans. . . but her heart is strong and she follows where it leads her.

About lifespan discrepancies, they're not always as big as you'd think them to be. There's a dunmer in Morrowind that says that members of his family usually become about 120-130 years old. It's usually mages and upper class dunmer that live very long, while commoners and laborers don't.

I found this on http://www.elderscrollsonline.com/en-us/news/post/2013/05/06/ask-us-anything-variety-pack-4 done by some Bethesda/Zenimax employees. And that's the first question they answered:

So, that means that any mage can match an elf's lifespan (or see it prolonged to no end if said mage is an Altmer, maybe?) if s/he's powerful enough, right?

I also recall there being some disdain in Anvil for a mixed marriage with a Bosmer in TES IV, but only for Bosmer.

That seems to be the case. For example, the famed Nord mage Shalidor was around to battle the Dwemer before their disappearance in 1E 700, yet is encountered in ESO, well into the Second Era (although it's worth noting that one of his supposed accomplishments was stealing the secret of life from Akatosh).

First Orcs are Elves not beasts.

Second, yes an elf mage can prolong their lifespan. The Dunmer mage Divayth Fyr was 4000 years old at the time of Morrowind in the 3rd era. He is one of the few Dunmer that remembers what it was like to be a Chimer. He was also older than the Tribunal, the three God Kings that ruled Morrowind for millennia. His current whereabouts are unknown, but if a low ranking Telvanni councilor like Neloth is alive, then it's safe to assume that a 4000+ year old mage who aside from being a Telvanni was a former Psijic, then he's still alive and kicking.

Low ranking....he is a master.

Maramal and Dinya for one, and she's pregnant FYI.

Yeah I know. I meant compared to Divayth Fyr's power ranking.

Yes. And it would seem to me that this could be more of a marriage consideration than life span differences.

I expect that Ulfric wants lots of strong young go-to-Sovngarde-when-they-die big strong Nord children to carry on his legacy. I expect that marrying a wood elf would not be something he would consider.

Similarly, I can imagine many Nord men who want plenty of strong little Nords running around and would seek a woman who can deliver that. Elves not only produce only elven children but are not as prolific as humans; despite their long lifespan, they are lucky to produce as many as two children during their lives.

Same for an orc chieftain in selecting wives to produce strong members of his tribe and a future chief. . . .

Even Eric the Slayer might want Nord kids to carry on his farming and adventuring instead of having little elves or kitties running around.

No, only remotely attractive mer in the game is Karliah, the rest of them, well, let's just say they fall too far into the uncanny valley for me.

Symmachus? I could have sworn that he was a Dark Elf.

After Symmachus, she married Kind Eadwyre of Wayrest. She also had some romances with humans before Symmachus.

Yeah, elf-human marriages seem to be pretty normal in the setting, although I'm sure there are racist or purist types that scoff at the idea.

Wasn't Symmachus rumored to be half Nord and half Dunmer? It is said he looked like a Dunmer but had the height of a Nord.

Though its worth mentioning, we never really see any bigotry directed towards Aldmeri/Human couples in Skyrim, despite it being well...Skyrim. Given how loose Nord's are supposed to be when it comes to marriage, I don't really think they care much for who beds who, most of the time.

The most we see in-game comes from the very anti-Imperial Vulwulfl, who is not satisfied with his son marrying one, does nothing to actually prevent it from happening beyond a little grumbling.

Tiber Septim probably being the most noteworthy.

TES5 Nords seem pretty accepting of anyone that assimilates into their culture, or at least makes some attempt, like Irileth. They're certainly not like the TES3 Dunmer, who were heavily prejudiced against anyone who wasn't a Morrowind-born-and-raised Dunmer.

Can't believe I didn't bring it up at the time, but one significant example of hating interracial relationships is most likely Alinor-born Altmer. Between the whole racial purity thing and a rigid class system, I don't see that ever really going over well. IIRC, there was an Altmer that mentions having a Khajiiti lover in a correspondence, and was basically disowned by their family for it. Don't quote me on that though, its been awhile.