The Feynman path integral Monte Carlo formalism has been combined with an ab initio configuration interaction approach in order to analyze the excited singlet states of the benzene isomers C6H6 and C6D6 under the conditions of thermal equilibrium. Electronic transition energies (PI)(E) over bar (T) and oscillator strength (PI)(f) over bar (osc) which have been sampled over large sets of nuclear configurations, are compared with single-configuration results E-T, f(osc). The latter set of quantities has been derived for the D-6h energy minimum of benzene. The present quantum Monte Carlo simulations lead to a simple physical picture for the nonzero intensities of the transitions to the two lowest excited singlet states of the two benzene isomers. Under D-6h conditions, the transitions to the B-1(2u) and B-1(1u) states are dipole-forbidden. The influence of the nuclear degrees of freedom on the excited-state properties of the two pi rings up to room temperature is quantum driven. The quantum fluctuations of the nuclei on a potential energy surface with large anharmonicities lead to a sizable redistribution of the transition intensities. At the same time they lower the transition energies (PI)(E) over bar (T) relative to the ET numbers at the energy minimum. Transitions, which are dipole-allowed for the rigid D-6h symmetry of C6H6 and C6D6 lose intensity under the influence of the nuclear fluctuations; vice versa for transitions dipole-forbidden under the constraints of the point group D-6h. The temperature and isotope dependence of these effects is discussed. Inherent problems of excited-state calculations of molecules on the basis of a single nuclear configuration are emphasized.