Leaving a note above the toilet saying "please flush" is perfectly reasonable and doesn't make a personal issue of this.
But the lack of hygiene, or the assumption that you, as the lone female, would provide it, is serious. You should have a conversation with whoever is the boss, focusing on the hygiene issue. Ask the boss what the company plan is for dealing with this problem. If he assumes that you are the bathroom custodian, ask him if he thinks that just because you are the only woman in the office. Stay cool while you ask this. It may dawn on him that he is making a huge mistake.
If it doesn't calmly explain that you do not have time to clean up after everyone else, and that cleaning the bathroom must be a shared responsibility, with assigned shifts, or that a cleaning service must be hired, because it is unacceptable to have to use a dirty bathroom.
It is not likely that this will make the boss like you any better, but at least he will respect you if you maintain a cool, professional approach. If he doesn't, you will have to decide if you want to continue working with people who treat you like a servant.