Hey y'all...
I think that should not be too hard of a fetish to fulfill. Of course it's within the vast realm of possibility that a woman would find her self impreginated by some errant rogue sperm making the trek in from the burbs (pre-pussy lips) but the chances are soooooooo slim that it's not really worth worrying about.
In a sense, many many guys Myself included indulge in BBBJ and there's way more risk for transmission of stuff than the risk of a lady gettin preggers.
I think if the woman was paranoid, she could put a little saran wrap under the panties, wear a tampon, wear a lil maxipad, hold her breath or other some such technique to avoid a small perversion such as this.
Other than that man, it's no big deal. If that is what turns your crank (and that's cool, I have a raging highly specific fetish as well) then just tell the lady you're with. Be nice and sweet to her and if you have a good relationship she'll be looking for ways to please you. If she knows you love to jak yer load electric all over her nicely modelled skiivies, I'm sure she'll go the extra mile.
I make requests about my specific fetish all the time. It's not gross, just specific, as is yers... and if they like me... and they always do (I'm a sweetie) they fucking ROCK MY WORLD
Catcha later
Sincerely,
Gawd Almighty