Firstly, one would hope that men would have a sense of unfairness at living in a society which objectifies and discriminates against women on such an insidious scale.
Secondly, one would hope that men would care enough about the women in their lives to want them to have equality of rights and opportunities, to have aspirations beyond being a glamour model or a lap dancer, to want them to be able to walk the streets at night safely, to want them to not feel pressured into feeling that they have to look a certain way which can lead to self-esteem issues/depression.
Thirdly, feminism benefits men because, as others have explained, it removes the gender stereotypes that apply to men and women and the subsequeunt pressure on both sexes to look and act in the way that is expected of their gender.
A small example of this is work clothing. The men in my office wear suits and ties all year round and complain about being hot in the summer. Women can wear suits/dresses/sleeveless tops and have much more freedom to be able to wear what they like.
There's no reason why men shouldn't be able to wear more relaxed clothes in the summer or makeup or have long hair but these things are all associated with the 'feminine'.
In addition, the objectification of women leads men and women to have unrealistic expectations of how women should look and behave (and yes, this does also apply to men but to a lesser extent). Reinforcing women's role as objects is going to affect a man's ability to have healthy, intimate relationships with women who have real breasts and body hair and personalities.
I worry about the younger generations' expectations of how women and men should behave and look. I do think this affects women more but it cannot be good to be a teenage boy who is going out into the world thinking that every woman he has a relationship with should look and act like a porn actress.
Plus everything else that everyone else has typed!
Have you checked out the Antiporn Men website? There might well be something on there.