Some moral permissibility facts are logically trivial: it permissible to eat a morally appropriate breakfast. And maybe some moral permissibility facts permit something negative where the permissibility derives from the impermissibility of the opposed positive action: it is permissible to refrain from murder.
But a non-gerrymandered, logically non-trivial permission of a positive action is always something contingent fact (assuming it is a fact), and depends on empirical knowledge of the world or revelation. It is never a priori. The reason is simple. For any non-gerrymandered, logically non-trivially permission claim about a positive action A, we can find a logically possible situation in which A has some horrible consequences down the road, consequences so horrible as to make A impermissible (I am not here assuming consequentialism—even the anti-consequentialist has to allow that an action can become impermissible due to its having disproportionately bad non-intended consequences) such that we only know empirically or by revelation that these consequences do not obtain.
Therefore, while one can perhaps engage in purely armchair discernment of obligations, one cannot engage in purely armchair discernment of permissions (except in gerrymandered, logically trivial or negative cases). Data about the world around us is always needed, either obtained empirically or by revelation.
In particular, unless one's knowledge of a non-gerrymandered, non-trivial permission of a positive action comes from divine revelation, such knowledge suffers from the kind of defeasibility that all empirical knowledge suffers from. We're not going to be dealing in the self-evident when we make these permissibility claims.
This should lead to a certain epistemic modesty when making claims such as that eating meat or engaging in homosexual acts or lending at interest is permissible, unless one has apposite divine revelation (I think in the eating meat case, we do in fact have divine revelation that sometimes the eating of meat is permissible; but it does not follow that in our day, affluent Westerners who can get nutrition from other sources are permitted to eat meat).