I'd forgotten that he doesn't receive any coin himself. He does come with several sovereigns' worth of high-quality leather armor and daggers though, probably worth more than Tabris' entire house, where Leliana has just her robes and a pair of knives. And as you say he is a working man with a very marketable skill, even if you kind of cost him his career. I'm given to understand that women appreciate that in men.
I think we all know that "trying to learn" and apologizing is almost never enough for people who are used to seeing you as part of an oppressive machine. "Your people deserve dignity? I never thought of it like that." is a hard thing to backpedal from no matter how quickly and genuinely you change your thinking. I didn't mean to say that Leliana is so ignorant, just point out that at least some tension there would be very realistic. And a lot wouldn't be implausible. And no, nobody is accusing Alistair or Morrigan of being more open-minded, although Morrigan's own estrangement from human society and cynical focus on independence could make her more compatible with, say, a Dalish elf's view on the issue.
Zevran, on the other hand, is both a fellow elf and one who's been around. He could probably discuss circles around Leliana, Morrigan, Alistair and Tabris herself combined on the subject without insulting anyone involved. All I'm saying is that's another argument for Tabris to be naturally interested in him over the others. He's both closer to home and exotic, on top of understanding what her people go through.
I think you've definitely got a point about him being "both closer to home and exotic," which is the best combination for some people. 
I have to disagree about the financial security part. As you all have said, Zevran himself notes that he is "as poor as a Chantry mouse, come to think of it." He also notes in conversation that he doesn't know what he'll do now that he's left the Crows, since the only two skills he has are assassination and... something he'd get arrested for performing in public (sex!). Sure, he could learn a new trade (I headcanon that my city elf encouraged him to), but those things take time too. I don't really think money would be an object, though, since you're part of the Grey Warden now. Assuming you survive the Blight and restore the Order, they'll probably pay your way, so you don't have to worry about it.
I can't speak for most real-life women since I'm only one of them; but, like with most things, it depends on who you ask. Some want to be taken care of, some just want someone whose career, hours, income and/or education level is somewhere in the ballpark of theirs, some just want a partner who'll help out (whether as "house husband" or also generate income), etc. I think most of us have to worry about food and bills in real life, so I think that desire for stability and financial security comes more from a desire to have a partner who'll help us through life, rather than someone who expects you to carry yourself and them.
Overall, though, I think there is something to be said for dating someone who's gone through the same things you have as one of the people.
Yes, yes, I know that's what you said. I had to emphasize it because you spoke as though my point were about staying stuck in the past.
The motivation aspect I brought up was related to his new pursuit in DA2. But if we're speaking about the first meeting in DAO, why would you believe Zevran's loyalty will last?
And of course, you assume an assassin would need to engage in open combat. Your party's ability to defeat him is irrelevant when he could just poison their food or silently kill them in their sleep.
Why would you trust Sten not to snap or kill anyone again? At the time that he's standing in the cage, we don't know what kind of trigger or madness led him to do it, and he doesn't exactly offer any details or have any intention of giving you confidence that he wouldn't kill again, so what if it struck again?
It's called trust. Different players have different reasons and motivations for trusting different characters. Reasons that might seem insufficient to one might be perfectly sufficient to another. As I already know you're going to dismiss any reasons I give for saying why that protagonist might trust Zevran, or feel confident enough in their own prowess to beat him if he tried again or smarts against falling for his tricks, I won't go into it.
We'll just have to agree to disagree.