Space-division multiplexing (SDM) in conjunction with elastic optical networks (EONs) can significantly enhance the network’s fiber capacity in order to accommodate the current capacity crunch. Nevertheless, SDM-based EONs pose new challenges for network operators in terms of efficiently utilizing the network’s spectrum resources. This work proposes a novel impairment-aware optimization framework for the routing, modulation, spectrum, and core allocation (RMSCA) problem in SDM-EONs. In the proposed framework, the concept of bit loading is used, wherein each frequency slot of a connection is modulated independently depending upon the actual interference, as compared to uniform modulation techniques used in existing works. Further, the proposed optimization framework jointly considers physical layer impairments (PLIs) of both inter- and intra-core effects. A mixed integer linear program (MILP) model is formulated to address the RMSCA problem in SDM-EONs, while guaranteeing an acceptable end-to-end quality of transmission (QoT) for each connection. Performance results demonstrate that by using bit loading in SDM-EONs, the network’s spectrum resources can be efficiently utilized.