The problem you have with your logic is that it's not just about the people who it impacts directly. Some of us care about the rights of others and/or think beyond themselves.
For example, if there is legislation banning a gay couple from getting married, what's really stopping legislate banning interracial marriages. You know, that whole slippery slope thing. Suddenly more people care. Then consider parents, who find some of these archaic ideals and even laws start impacting their kids. Now even more people care. Then those people have friends who support. Now it's more.
Tampons in boys bathrooms is meaningless. It doesn't impact you, even if you're in there.