Ernest Belfort Bax was an early socialist who wrote more than 20 books about philosophy, socialism, history, and various other topics from his time period. His work was featured in several prominent left-wing socialist magazines and periodicals, including the Social Democrat.

He also wrote several books about men's rights and the early women's movement. According to him, women living in Britain were already privileged, and it was men who needed special consideration when it came to gender equality. He discussed in depth some of the gendered laws and social standards surrounding marriage. As well as the treatment of men and women in civil court, criminal court, and family court. He even claimed that there was a media bias against men in the newspapers, especially with regards to domestic violence. Women were believed by default, and men who were the victims of female violence were laughed at and ridiculed.

For example, in The Legal Subjection of Men, he writes:

In the first place a woman has only to complain against a man, and the tribunal is already convinced of the justice of her claim. The tribunal is only impartial if the complaint is by one woman against another. In the next place, no adequate repression of crime or other injury by a woman against a men is even attempted.

This tendency of the tribunals is confirmed and rendered irresistibly by the actions of the press and public opinion. All injuries to a woman are chronicled with flaring headlines. Injuries by women to men are laughed at, or worse still, passed over in silence.

He also believed that the only way men could be equal to women is if we adopted socialist principles in society. According to him, women were using capitalism to exploit men and male labor. A socialist economic system would force women to work just as much as men because under socialism, everyone is treated the same, regardless of race or gender.

From the same book (original emphasis):

Certain Socialist writers are fond of describing the Social-Democratic State of the future as implying the "emancipation of the proletarian and the women". As regards to the latter point, however, if emancipation is taken to include domination, we have not to wait so long. The highest development of modern capitalism, as exemplified in the English-speaking countries, has placed man to all intents and purposes, legally under the heels of women. So far as the relations of the sexes are concerned, it would be the task of Socialism to emancipate man from this position, if sex-equality be the goal aimed at. The first step on the road towards such equality would necessarily consist in the abolition of modern female privilege.

I'm not saying I agree with everything he wrote but it's definitely interesting form a historical perspective. And it shows how leftist ideas, including socialism, can intersect with men's advocacy.