When starting a new character, you get to chose your race, gender and portrait at the first screen.
Unless you want to play as a dwarf, gnome, halfling or half-ogre, in which cases you can only play as a male character.
Is there any particular reason to this, or is it just an oversight from the development team?

It was a developer decision made to limit the number of sprites needed in-game. Note that all of the playable female PCs are medium-sized. The only small female sprites anywhere in the game are the NPC Gnomish noblewomen wandering the big cities like Tarant, and there aren't any large females anywhere (unless of course you count the larger-than-life Silver Lady). Remember that each sprite needs not only a basic outline, but also a different appearance for each type of armor, weapon, and whether they're carrying a shield.

There are also halfling NPC females - some magic traders ("gypsywomen"), Mrs Halster in Tarant, farmer's wife in Ashbury, etc.

It is also explained lore-wise as dwarven women being very secluded because of poor male to female ratios, and insanely long pregnancy times; halfling female lack of adventurishness (I think that is what the reasoning is), and the extreme rarity of half-ogre women in general.

I stand corrected. Still, those are one-off characters; you never see Mrs. Halster in battle dress (or even a fancy dress) for instance.

I remember how sad I was when I realized that it's impossible to create half-ogre woman :(
There was one in Tarant I think.

where?

Maid in some tarant mansion if my memory is correct, but I can be wrong tbh :)

Only if it was a crossdresser, there is no female halfogre in the game

Oh, that's right! It was in Caladon; you could interview her as part of one of the in-town side quests.

Yeah, she's a half-orc.

If you say so, but I'm sure she had different (bigger) model than my half-orc female so that's why I got confused... I think.