Responding initially to the title of your question, before reading anthing else:  STDs are rarely if ever transmitted by hand-genital contact, even when genital fluids are used for lubrication. STDs are not simply infections that happen to involve the genitals. The bacteria and viruses that cause them evolved to require large doses for transmission, and/or to require access to certain cells, both of which dictate sexual transmission. In fact, they are STDs precisely because they organisms, and the human immune responses to them, evolved together to require this -- which is why they are STDs in the first place. It is rare if ever that sufficient virus can be carried over or be transmitted without more direct sexual contact than hand-genital exposure. So unless there is something surprising in the details of your question, I am confident you are not at risk.

Now I have read the question itself. Guess what? First, thanks for reading other discussions ahead of time. Most likely you predicted the information above, and the details don't change my opinion or advice. Second, congratulations on having entirely safe sex. Third, I would add that most escorts -- i.e. expensive female sex workers by appointment -- tend to have low STD rates. After all, their customers are typically low risk -- i.e. men like you -- and most are aware of STD risks and their prevention and also get tested regularly. So the odds probably were low your partner had a transmissible STD.

As for "penty of vaginal fluid that came into contact with [your] penis", that happens all the time anyway, during condom protected sex: the skin of the upper part of the penis usually contacts vaginal fluids, usually with no STD transmission.

Thank you for your prompt response to my question. It's an honor to receive such detailed explanations from you, as I have always admired the responses of yours found on the internet.  I'm also glad that your answer on this subject will benefit other 'anxious' persons like me, who I'm sure are seeking the same satisfying explanation I received today. 

Your observations on this are more than reassuring and while I may find some room for a few "what ifs" in this follow up, I don't think they are so relevant. I don't think they would succeed changing your opinions either :) 

Well, maybe, let me make sure that if a similar exposure should happen again in the future I don't feel like playing the Russian roulette. What I mean is that if this kind of incident should happen again in the future (with different women), I can be certain that any vaginal fluid (regardless of the amount) coming into contact with the penis during masturbation is "not" a vehicle for STDs/STIs. Maybe one tiny question I will add here, without wanting to elaborate too much:  is it really so crucial that the vaginal secretions have to enter the penile urethra for an infection to take place, or it is just more important to bear in mind that vaginal fluids cannot infect via hands?   

Thanks so much again!!

Some STDs are transmitted primarily through genital fluids, such as chlamydia, gonorrhea, HIV, and hepatitis B. For those, secretions would have to get into the urethra. Others are transmitted primarily by skin-to-skin contact, and probably more transmissible by exposures like yours. But both are low risk. However, However, "cannot" is an absolute term, and if you're asking for a guarantee that such exposure could never result in transmission, I can't give it -- for either type of infection. Nobody can prove that such exposures will never result in STD transmission, only that it is very unlikely. If you want 100% protection, you will have to forego such sexual choices. 

That completes the 2 follow up questions and replies that come with each new question, and so ends this thread. The bottom line is that you didn't have sex, in the conventional sense, so there was little or no risk of STD. All the other details really are superfluous.

Take care and best wishes. Stay safe.