tbh I don't think it has anything to do with the fact that these people think that mn are Anti men either but that they're doing it because they can and nothing more, but Anti man makes for a good headline/agenda iyswim.
However I do think it slightly disingenuous to suggest that the same applies on here to both men and women, because while there is definitely a lot to be said against women who try to withhold access with ill intent or out of revenge etc I don't think it can be argued that in many cases there are definite double standards in terms of how posts from men and women are answered on mn. e.g.:
woman posts that her dh won't have sex with her: replies are along the lines of she should reconsider the relationship, would she want to stay in a sexless marriage for the rest of her life etc, quite valid points IMO. But if a man posts the same the initial question is how much does he do to help around the house. 
The thread the other day with the man who doesn't want any more children because he has a severely disabled child from a previous relationship as well as his two year old is a case in point. There were two posters in particular who were particularly vitriolic and who were rightly called on their posts, but also a number of others suggesting that the man should perhaps come up with solutions to enable the woman to be able to have the baby she wants. If the situation had been reversed there is no way that anyone would have said anything other than that it was her body, her choice, end of discussion.
I don't think that mn is anti man, but I do think that because it is predominantly women posting here there is more of a leaning towards the opinions of women being the right ones vs those of men.