It may be in opposition to a base biological function, however the description says in relation to MENTAL functioning, which this is not.

When you get into things like sexuality, you have to take into account hormones, testosterone and estrogen levels, things like that, which can directly effect someones mental state. So something like this kind of falls under both categories, but I'll have to say that sexual desires and preferences are largely a mental thing.

They are mentally sound and the co-morbidity is likely to be as a result of social cues as you mentioned. I would be interested in the data on depression in LGBT for example across countries with various acceptance levels of LGBT.

I would absolutely like to see this kind of data too. But you also have to take into account that if improper hormone levels/neurotransmitter levels/etc. can cause problems like depression, anxiety, etc., then if homosexuality is caused by an imbalance in these things (which research points to being true), then any imbalance compared to the relative "norm" could put someone at a greater risk for various other issues.

Under your line of thinking, people who choose to not have children, which as /r/childfree indicates is not uncommon, are mentally ill? Just because something is a baser instinct via evolution, human beings have agency unlike any other being, so I don't think you can consider it to be an illness when they deviate from it.

I touched on this in an earlier comment, where I rephrased what I said to focus on the fact that I was more referring to someones "primal sexual urges/desires" as opposed to their actual desire to have and raise children. But you're right, someone deciding not to have children does not constitute a mental disorder. I just phrased myself poorly.