In a world were STD tests cost absolutely nothing, including time, effort, thought, there would be no excuse to not have taken a test and I do not see a method for generating plausible deniability by not knowing.
That’s not really the least convenient possible world though, is it? The least convenient possible world is one where STD tests impose no additional cost on him, but other people don’t know this, so he still has plausible deniability. Let’s say that he’s taking a sexuality course where the students are assigned to take STD tests, or if they have some objection, are forced to do a make up assignment which imposes equivalent inconvenience. Nobody he wants to have sex with is aware that he’s in this class or that it imposes this assignment.
That’s not really the least convenient possible world though, is it? The least convenient possible world is one where STD tests impose no additional cost on him, but other people don’t know this, so he still has plausible deniability. Let’s say that he’s taking a sexuality course where the students are assigned to take STD tests, or if they have some objection, are forced to do a make up assignment which imposes equivalent inconvenience. Nobody he wants to have sex with is aware that he’s in this class or that it imposes this assignment.