The impact of loss on the plasmonic resonances in metal-insulatormetal slits is analyzed, particularly the significant effect of loss on the reflection phase. The reflection is calculated analytically using single mode matching theory with the unconjugated form of the orthogonality relation. This theoretical calculation agrees well with comprehensive simulations, but differs substantially from the conjugated orthogonality result, as was used in past analytical works. This reflection phase has a large impact on the plasmonic resonance wavelengths, which are calculated using a Fabry-Pérot theory and compared with past experiment and finite-difference time-domain simulations.