There are others that also thought/showed a possible link with STD, but it remains unclear whether this is really case. Anyway, thanks for sharing.
If you have this problem long enough, then at some point you will think about everything, ranging from food, allergy, sexual activity, weather, microorganisms etc.
Personally, I think that it's likely that there are at least two factors involved, of one which is (I think) a microorganism (such as demodex and bacteria), the other may be genetics, allergy or maybe another microorganism.
Also note that demodex is transferred via skin-to-skin contact or indirect contact (via object touched by someone else, such as a towel). The highest transmission of demodex is via body-to-body contact, so if you have oral sex, you likely transfer more demodex than simply shaking someones hand. So it may not be oral sex specifically, but more generally skin-to-skin contact. (Combined with another unknown factor.) This also gives a higher transmission of other microorganisms, such as bacteria and viruses.
But @Wiljon, maybe you could try Soolantra, a relative very high number of users here have success with it: http://oozingyellowcrusts.com/t/soolantra/64
If nothing really helps, then you owe it to yourself to give Soolantra at least try.