Any virus needs an entry point, and the stripper slide has more friction on delicate areas.
The mouth and vagina are both moist areas and the tissues are very similar in composition. It's more the mechanics/logistics that increase the risk.
The mouth is almost always moist and gets wetter with insertion of anything, food or not. Thus a BBBJ will increase salivation and this aids in minimizing friction. Also, the mouth is sensitive in that it is filled with nerve endings. You will likely feel any friction and/or will be aware of any sores before/during the act. Additionally, the delicate cheeks are rarely involved in the act. Tissues tend to remain intact, and any potential virus has no entry point. I recommend not performing BBBJ if you have brushed/flossed within the last few hours or if you have had any dental work done in the previous few days.
The vaginal area is also moist, but it's more temperamental. The stripper slide is on the external tissues, away from the vaginal lubrication, and this can cause micro tears you may not be aware of because of the far fewer nerve endings in the general area (save for one or two very tender areas). The urethra is also a very tender area and it is smack dab in the center of the action. This is why UTIs are very common for women who are very sexually active. The area is easily overstimulated and can become inflamed, allowing bacteria easier entry to the canal, but I digress...
Ultimately, even if a person has HIV, Hep, HPV, there needs to be a break in the tissue for the virus to do anything. The stripper slide just increases the chance of that happening.
One other thing, shaved/shaved is also increasing the risk, as the skin/skin friction can cause irritation and breaks in the skin. HSV can easily spread even without this added risk. As such, I highly recommend trimming/grooming instead of fully shaving the area. It's just much safer for all involved, especially the woman.
(yes, I'm a medical professional