I'm a strong believer in safe sex so if I write a sex scene there is most usually a condom involved and if there isn't there probably was a mistake I made in my writing, but I am pretty certain that all of my sex scenes ever written have had safer sex practices.
As far as your questions, I believe some people stray away from safer sex because they want to, others just don't think about it, it's not that they don't want to write about safer sex, but it's just in the middle of the passion and the relationship taking another level and whatever they just don't want to pause it to write in the "necessaries." So maybe why people don't like writing about their characters going to the bathroom and such could fall under that as well.
But I do agree with you, safer sex in stories are a good message to make since people read them, but I don't think people will try to emulate the characters they read about here and not have safe sex. As far as selling safer sex I don't thin that's an author's responsibility.