Well you really have to go super wild to get a STD between girls. I mean, sure, they do distribute those safe packs in my usual bar, but to be honnest the only thing I got sleeping with random girls without protection, so far, is lice in my hairs due to an ex having kindergarden nieces.
Not saying this is impossible or that you shouldn't protect yourself. But the chance is still pretty low.
As far as this manga goes, well, it's Kurogane Ken. It's not bad, it's just not good. This manga especially was obviously a no-scenario x-rated excuses since the very beginning so it comes as no surprise.