A general approach based on the Parsons-Lee theory for soft repulsive molecular fluids is employed to investigate the nematogenic behavior of prolate thermotropic liquid crystals over a broad temperature range. The theory is solved for the particular case of the Kihara soft repulsive spherocylinder model, which is mapped into an effective hard core interaction with a temperature-dependent molecular diameter, expected to resemble the average size and shape of the soft molecules at a given temperature. The reduction of the effective molecular diameter with temperature in the Kihara soft repulsive fluid implies implicitly an increase of the elongation of the molecule and induces the stabilization of the nematic phase at smaller effective packing fractions, contrary to what is found for other fluid models. The rationalization of this effect in terms of excluded volume steric arguments is corroborated by the good general agreement between the Parsons-Lee approach and Monte Carlo simulations for the equation of state of the fluid in the vicinity of the isotropic-nematic transition.