I forgot about this, which now leads me to believe that Solas wanting to have a child is even more unlikely. If you can keep playing, then you're not in the process of settling down to be pregnant.
I'd like to throw out questions so we can brainstorm what the story reason would be for an elf. Critical thinking is my thing. 
1. Why would an elf want to be with another elf?
2. Why would an elf not want to be with a human, a dwarf, or a qunari?
3. Why would an elf not be able to be with a human, a dwarf, or a qunari in spite of his desires?
4. What are the benefits of a hedge mage elf coupling with a Dalish elf (of any class) who is also leader of an Inquisition and the Herald of Andraste?
5. What benefits does the fem Dalish Inquisitor receive for romancing Solas?
6. How does religious/nonreligious affiliation tie into Solas' desire for a female elf over other races? How does that tie into his Fade knowledge?
7. Could it be possible that Solas has previously met the fem Dalish Inquisitor? (And of course she has no memory of this) Possible "we-have-previously-met" tropes: a female child he once saved in his youth who stole his heart, an old flame, an old friend, an old enemy, an arranged marriage... (Any other tropes?)
Time to answer.

Hmm, I'll have a go at these. Just speculation, but fun questions.
1, 2 &3. An elf may simply only be romantically attracted to those of his own species. Not that he/she dislikes other species, they just don't see them in that way.
Or, perhaps Solas (specifically) is afraid that because he has spent so much time dreaming in Elvhen ruins that his lucid dreaming may negatively impact an individual of another species if he becomes intimate with them.
Or, as has been brought up before - lifespan issue associating with other species if Solas is an ancient elf or has discovered secrets relating to their long lifespan.
Another point that has been brought up often, if the union results in children he wishes them to be Elvhen.
Maybe there are some aspects of intimacy that are different if two elves are involved - possibly something related to Solas' lucid dreaming, a sort of mental bonding?
Or maybe Solas' abilities would diminish or vanish with intimate association with another species and he views his unique research and perspective as more important than his personal desires (sort of a "Tale of Two Cities" needs of the many outweigh the needs of the few or the one).
4. Protection from Templars (if the mage/Templar conflict is not resolved in favor of the mages).
Political support for his perspective on the fade - perhaps even promotion of his ideas about teaching people not to be afraid of the fade.
Political and possible military support for his research - he may be able to explore ruins that were previously out of reach. Perhaps they were to close to inhabited areas or to dangerous for him to explore alone.
Aid in closing the Fade rifts and saving the world.
If the Inquisitor is known as the Herald of Andraste (which may not be true in all world states, we will have to see), then he will also have religious backing to support his ideas.
5. Intimacy with one of her own species who has knowledge of Elvhen culture.
Increased insight and knowledge about ancient elves.
Potentially a more "special" kind of relationship than can be obtained with a non-elf.
A mate who would be acceptable to her clan and who she could have Elvhen children with if that was what they both desired.
A LI who is as fascinated with unearthing ancient Elvhen culture as she is (for some Inquisitors).
6. Solas may worship the Elvhen pantheon and could not imagine intimacy with someone who doesn't.
Solas may be aware that all religions are based on false information, but he prefers to be in a relationship with someone who does not believe that converting an entire continent is necessary.
Solas may have direct association with one of the Elvhen gods and be unable, due to that association, to be intimate with another species. Commandment of the God or something.
7. I find this one the most problematic, as it interferes with the player's agency. It is possible, but I suspect if there is a connection it is made when the Inquisitor falls into the fade. The concept of a previous association could be interesting, but I do think it likely the developers would have found a compelling reason for it (and perhaps they did).
Edited for formatting and one bit of awkward phrasing