Anorgasmia during pregabalin add-on therapy for partial seizures.
Those for whom pre-prints are really important sources have already moved beyond that anyway, but I know that varies heavily by discipline.
A second reason to think sexual rhythms can entrain brain oscillations is their multi-modal richness.
Can be as broad or narrow as one likes.
A 2002 study by Chudakov et al.
In theory, increased copulatory-phase theta activity could indicate the establishment of a network of enhanced pre-orgasmic functional connectivity, with subsequent spindle activity indicating the learning of partner characteristics from associated sexual stimulation.