I'm not a man, but mentioned in another comment that manspreading is more about comfort due to having testicles and I can sympathize with that. If the dude is significantly impairing my ability to sit straight on the seat, I will usually spread my legs a little wider to force him to move a little, but if not-- if he's just taking a bit of room-- it doesn't matter so much.
I don't think it's a problem that needs to be solved. Imagine if your discomfort was visual: you're walking on crutches or you have a cast. People would naturally give you more room because they don't want to cause you any pain. Men are not about to say, "excuse me, ma'am, my balls are sweaty and it's not appropriate to adjust them to a certain degree in public, and besides that I don't want to crush them between my legs, so if you don't mind, please forgive me if I take up a bit of room on the seat."
I don't think it's a problem that needs to be solved. Imagine if your discomfort was visual: you're walking on crutches or you have a cast. People would naturally give you more room because they don't want to cause you any pain. Men are not about to say, "excuse me, ma'am, my balls are sweaty and it's not appropriate to adjust them to a certain degree in public, and besides that I don't want to crush them between my legs, so if you don't mind, please forgive me if I take up a bit of room on the seat."