I'm sure the other kingdom that treats the men like 'slaves', will actually have fairly low demands, like "you WILL have sex", I doubt a whip or chain are involved at all. The MC will probably charm that entire kingdom by being a basic bitch 'nice guy', and make them appreciate men and treat them like gods like in the FMC's kingdom simply by being a half decent non-self absorbed guy, and thus stopping the war... and maybe he'll change the minds of the men from that kingdom as well, by convincing them that women aren't that bad?
Of course, this is manga, so it's totally possible that the 'le bad' kingdom cartoonishly tortures the men (probably while the women make 'le evil' faces, with their tongues sticking out), despite the risk of having all the men flee and kill themselves, thus lowering their male population even more, but logic never applies to these things anyway.
