We can make up explanations like prostitutes and whatnot, but we have pretty good evidence that it's simply women lying.

I've lost it, but I once ran across a paper that surveyed prostitutes and concluded they explained the entire difference. Most anomalies in social science that people are willing to admit exist in the data are explained many times over by lone papers. Sometimes even by whole bodies of literature that fail to acknowledge the other school of thought.

(ETA: here is a paper along these lines. here is a popular account. This paper is really not about under-sampling prostitutes, but about how prostitutes massively underreport.)