I mean, let’s assume for the purpose of argument that it has been found that men receive more time for similar crimes. It does not therefore follow that a) this is a feminist issue, and b) this is an example of “sexism against men.”
In any system of oppression, there are going to be ideological inconsistencies that do not always work out in favor of the oppressor. The patriarchy tells us that men are strong and women are weak, and as a result, men are encouraged to act more aggressively, which leads the obvious problem of violent behavior and being jailed and so on. Feminism aims to end sexism, namely the institutional and systematic discrimination against women in society. It is not the responsibility of feminism to be concerned with every instance a gender discrepancy appears in society, especially if that situation is not relevant to female oppression. An analogous position would be to expect black civil rights leaders to oppose the recruitment of black athletes because it’s rooted in a racial stereotype, and civil rights leaders are opposed to racial stereotypes.
Secondly, there are a variety of reasons it might make sense for men to be given more jail time, independent of patriarchal assumptions about masculinity and femininity. The purpose of the penal system is not purely retribution, but also rehabilitation and prevention. If it was shown, for instance, women who served shorter jail sentences had lower rates of recidivism than men, or that increasing the average length of jail sentences had no effect on reducing the amount of violent crime by women, it would make sense, from a purely economic standpoint, to give women lighter sentences because you don’t want your jails full of people who don’t need to be there. Furthermore, a lot of violent crimes committed by women occur as a result of battered women syndrome, where the perpetrator has a history of being abused, and finally snaps and responds violently to her abuser. It would be insane to treat a woman who kills her abuser the same way you would treat someone who murdered a stranger. A better way of reducing the violent crimes of women might therefore be to target domestic violence in general, such as by increasing the number and availability of resources for battered women.
Of course, this level of analysis is completely beyond what I would expect of the average MRA, who just wants to blame feminism for all their societal ails, even though society isn’t run by feminists.