Sankaku Channel/

Straddling is a posture tag so is correct, it is not possible to be in that position without any straddling.

I looked at girl_on_top whilst checking the translations and it is intended to cover girls on top of people in whatever context (one that makes good sense is a girl lying on top of another girl, which is not "cowgirl_position").

girl_on_top is not an appropriate implication because it may not actually be a girl on top.

vaginal_lubrication does indeed sound awful (it is from Wikipedia). vaginal_juice, vaginal_secretions... it is surprisingly difficult.