Although I cannot find a source, I believe it was stated that the BDSM community is composed of roughly 30% women and 70% men (feel free to discount a few points from each for non-standard gender distribution as required).
Now, given that there are no women on the internet, it makes sense that the population of female dominates is lower than male submissives.
But back on base, what you are seeing is the results of the capitalist system in play. Specifically, the laws of supply and demand, which can be used to show that a low amount of supply with a high amount of demand will make people do some crazy things to become supplied.
The shotgun approach of replying to any thread at all which has a female posting looking for something with a heartbeat (an optional requirement when really hard up on the weekends) ends up giving them a higher chance to meet their demand and obtain some supply.
That isn't to say that it is a good way, the right way, or the best way to do as much. It is none of those things. But it is an easy way, and easy things are evolutionary optional when dealing with the guided evolution of internet community patterns.
There are way to solve this, such as making posting a reply in a thread for someone not searching for your gender/orientation/etc an offense that could get you a warning/short ban, although in which case you'd have issues with people posting references for the person.
And that doesn't clean up the issue with being PM spammed, so it really is not worth the effort most likely.
Might you, or anyone else, have any possible solutions to deal with the people that won't deal?