Why are the Turians and Krogans attracted towards the Asari sexually? Both Turians and Krogans are Bi-Gender species and have their own kind of females to get attracted to. The sexual attraction of humans towards the Asari seems valid beacuse Asari are almost similar to Human females but why do Krogans, Turians and other Bi-Gender species in Mass Effect get attracted to the Asari?
Simple answer: why not?
That aside, it is implied in Mass Effect 2 that Asari in some way affect other races, causing them to view Asari as being very attractive and similar looking to their own race.
At the Eternity bar, a conversation occurs between a human, a salarian, and a turian who are watching an asari dancer. The men debate whose species the asari most resembles. Upon each forwarding their own race, the human theorizes that the asari might be using mind control to appear attractive to other species. It is more likely that the three men were merely focusing on characteristics their species shares with the asari (e.g., body shape for humans, skin color for salarians, head fringe for turians). All three men compliment the asari's flexibility and grace. This conversation implies that asari are considered attractive to many species, which would prove useful considering their method of reproduction. Mordin Solus postulates that the asari's cross-species attraction may be neurochemical in nature. Asari:Biology
For some males, however, it may be that the Asari represent their only realistic opportunity for children. Granted, these children won't share any genetics with their father, but for some that won't matter. In Mass Effect 2 there is a Krogan attempting to woo an Asari on Illium, and Shepard suggests that he may be doing so because he can't have offspring of his own. Living with the genophage, it would be unlikely that an individual Krogan would ever get to produce Krogan offspring, and so they may turn towards Asari to fill that desire.
Here is the aforementioned conversation on Illium:
SALARIAN: I still don't see why we're here. Salarians do not "get married." My family simply negotiated a reproduction contract--
HUMAN: Whatever. It's the closest you guys get to a wedding. And that means you get a bachelor party. End of story.
SALARIAN: I don't understand. Humans celebrate wedding contracts by tempting infidelity? That makes no sense!
HUMAN: Calm down, man! Don't embarrass me in front of the stripper.
SALARAIN: What do turians do? Do turians do this, too?
TURIAN: I'm just here for the drinks.
SALARIAN: You said that bachelor parties are for very close friends. We're just co-workers. HUMAN: We've been co-workers for five years, though. Aren't salarian years like dog years? SALARIAN: Okay, now -- that's offensive.
SALARIAN: I appreciate the gesture, but my people don't even have sex drives the way humans do.
HUMAN Uh-huh. Take a look at that, man. Everybody likes the asari. Everybody.
SALARIAN: But we actually reproduce by... my word, she is very... limber. I can appreciate her dancing in an aesthetic manner, but... I don't have... feelings of... hmm... she is a lovely shade of blue.
TURIAN: Okay, that makes my legs hurt just watching it, and my knees are meant to bend that way.
SALARIAN: This isn't as bad as I had feared. It's actually... i-is it warm in here?
HUMAN: The little divot in her abdomen? It's called a belly button. Humans and asari have them, and you're going to be doing shots out of it later tonight.
TURIAN: That can't be sanitary.
HUMAN: Not the point, man!
SALARIAN: I can understand why I might find asari attractive. But how can they be attractive to humans, too? They look just like salarians!
HUMAN: What? They look exactly like us! I'm... I'm not seeing the salarian thing at all. At all.
TURIAN: You're both wrong. Asari look just like blue turians. Look at the head-fringe!
HUMAN: Wait. You don't think they're, like, mind-controlling us to see them as attractive, do you?
TURIAN: Please be quiet. You're going to ruin asari for me. And there aren't many turian women on Illium.
SALARIAN: Well, I suppose watching just one more dance wouldn't hurt.
HUMAN: Ha-ha-ha! That's the spirit!
TURIAN: As long as I'm not paying for the drinks.
Asari can get children with every one, according to one Turian there aren't many female Turians on Illium, there aren't many female Krogans anywhere off of Tuchanka. Asari have attractive features from various species: Human-like body, Salarians like the "lovely shade of blue", Turian fringes, scales, etc. I don't see anything for Krogans though, maybe lifespan, availability and ability to have children. Or the Human in the bar could be right about the mind tricks.
Not all features of an Asari are attractive to other races, some might even be put off by some of the Asari's physical features.
"Asari have those funny bumps that get in the way."-deleted line from the citadel groundskeeper
"Asari are so squishy, where are you supposed to get a descent grip?"-Friend of presidium fish krogan
It may just be evolution/genetic mutation that gave them these traits: Asari can mate with any species, even there own (though these pure asari are considered of lower worth - afaik, Liara T'Soni is one of those). They look for strong potential mates who possess certain qualities (intelligence, strength, etc...) to pass them on to their offspring.
Over the years (read: millenia), certain typical traits from the other races probably came to be a part of the asari physique (intelligence, blue color, belly button as 1loudninja pointed out).
Since humans relatively new to space exploration (other races have been doing it for thousands of years), the transformations humans passed on would be relatively minor for now (yet they are the most human-like aliens in the ME effect afaik). Because of the, I suspect that they probably started of as mostly humanoid looking aliens.
This is a very late response, but here is my idea: Asari seem to be physically attractive to some of the species in the game - Turians and humans, mostly, it seems. Salarians usually stride for their own race, same with Krogan and any other race in general. Anyway, Asari have head fringe which resembles that of a male Turians, they also have blue skin - which from what a Salarian on Illium mentioned resembles Salarians (IDK) and then you have their body shape (faces included) which resembles humans.
I don't think they alter their appearance any to appeal to other races. They just have certain parts of their body that appeal to one more than they may to another (if that makes any sense). I'll also point out that in the Mass Effect games Asari are held as sort of a sex symbol from their dancing to their, uh, 'helping with personal problems'. Asari do live longer than most species (besides the Krogan), so their physical appeal to many other species benefits their offspring, has to do with passing on good and beneficial traits that they can't receive from two Asari parents (supposedly), it's to make sure their race continues to excel.
But then again, they might just have the ultimate mind control powers.
The comment with the most votes is wrong.
I'm playing the game right now and I just came across the scene where the turian human and salarian are having a bachelor party. Halfway through the repeated dialogue options the human says,"look at that right there." Salarian replies,"the little divot in her abdomen." The Human says,"Yes, that's a belly button, it's unique to asari and humans,and we'll be taking shots out of it later tonight!" The turian then says,"that can't be sanitary."
Later they state that they see the asari differently the human stating it looks human the turian stating it looks like a turian etc.
Yet, the first dialogue makes the second one completely illogical since they can all see the "belly button" state by the human (a characteristic that is stated as being completely unique to both the asari and human species).
So, maybe I'm wrong and mass effect intends to have the asari control their images, but I just don't believe that's true.