Well, the area on the posterior leg opposite the knee (heh) is the popliteal fossa, but I don't think Google even has any solid entries for this form of non-penetrative sex. :-/
Strange, since there are many references to "axillary sex" (in armpit) and "intercrural sex" (between thighs).
Yea, the "kneepit sex" is strange enough... but did someone noticed that her pussy is RED??? is it something like: Oh My God I'm having my period, so fuck my kneepit!?