All the main characters sexualities are heterosexual. All romances, sexual encounters, lustful longings and marriages are heterosexual.
So why did Riker go after an individual who was from a species without gender if he only goes for chicks?
Because Soren was a female.
Have you even seen the episode? They very specifically claim that the J'naii only have one gender.
Soren: "I am tired of lies. I am female. I was born that way."
Soren was in fact a female, she lived in a society that had officially done away with traditional gender identities, members of the population were require to conform to the cultural norm of not personally identifying as either male or female.
But despite of their androgyny, the J'naii did have the two genders of male and female.
Soren wanted to be openly feminine, and not androgynous. Through the years she selected J'nailan males for her lovers. Soren's crime within her society was that she wanted to manifest a "traditional" non-androgynous female identity.
This is why she was "reprogrammed" by her society, the official position was that the J'naii had evolved beyond gender.
No they didn't. Soren may have been emotionally and psychologically a female, but she didn't have a vagina. There are no J'naii with vaginas, none with penises.
From the
script:
The J'naii are a genderless species; Soren and Krite, like the others, possess no demonstrable sexual characteristics.
And
SOREN
"He?" Commander, there are no "he's" or "she's" in a species without gender.
And
RIKER
A little. It's hard to grasp the idea of having no gender...
SOREN
It's just as hard for us to understand the strange division you have in your species... males and females.
And
SOREN
Just because we don't have gender doesn't mean we don't have conflict.
And
SOREN
The idea of gender... it's offensive to my people. You see, long ago we had two sexes, as you do. But we evolved... into a higher form.
(beat)
I don't mean to sound insulting... but on my planet we've been taught that gender is...
(struggling for the least offensive word)
...primitive.
And
SOREN
Occasionally, among my people, a few are born who are -- different. Who are throwbacks to the era when we all had gender. Some are born with strong inclinations toward maleness... and some have urges to be female.
(beat)
I am one of the latter.
And
SOREN
On our world these feelings are forbidden. Those who are discovered are shamed and ridiculed. Only by undergoing psychotectic therapy and having all elements of gender eliminated can they become accepted into society again.
And
SOREN
Tell me... about males. What is it that makes you different from females?
If this doesn't make it clear that there are no different genders for the J'naii, I don't know what will. Your argument seems to make about as much sense as claiming that since a biologically male transgendered person identifies as a woman, this person magically has a vagina.