Don't get me wrong. It could well be just a skin tag. I am not saying that it is one thing or another (i.e. my use of the words 'looks like' instead of 'is'). My only point is that when I see something other than smooth, unblemished skin I would not continue with the punt as a precautionary measure, that's all.
Why not spend a little time on google images and get yourself aquainted with what warts and herpes actually looks like. No one has perfectly flawless skin and less so in the genital or anal area. small moles or skin tags are quite normal and if prossies turned down every man with a skin tag or mole in that area most men over 50 would never get a punt again.
If a prossie shaves she is likely to get blemishes or bumps or even sore areas due to ingrowing hairs but you would put all of this down to herpes or warts so you will be missing out on a lot of very good ladies. You will also find your phone number shared around and yourself getting a rather bad name as a timewaster if you walk every time someone has the slightest imperfection.
As for trying to get to have a good look before paying. That will never happen. Its bad enough with those that try to get a kiss and grope and then after say they forgot the money but imagine how many would come along have a good old play and then leave before paying to go and have a wank instead.
If you put every blemish down to an STI you are in the wrong hobby