For me, it depends on the story’s sub-genre.
In a fantastical setting, I don’t usually mind if safer sex is not mentioned, because in science fiction or fantasy the issue can be easily covered by the worldbuilding (everybody has an injection! everybody has a spell!) even if the author hasn’t mentioned it explicitly.
In historicals, I wish there was a bit more worrying about safe versus not-safe thoughts, but again I’m a little more accepting if safer sex is left out. However, I definitely appreciate it when historical characters think about the issue, even if it’s only in the first intimate scene and left to the reader’s assumption after that.
In contemporary novels, I prefer that safer sex be practiced, and if it isn’t, that a reason I can accept is provided (obviously, not every writer can read my mind!). I don’t mind if subsequent sex scenes aren’t shown as safe, at least not so much, because I can extrapolate from scene number one, in much the way that I extrapolate the characters are eating, sleeping, and using the toilet even though those actions aren’t necessarily described.
What do you think?