Please for fucksake...

Stop typing!  Get some glasses too....The op never said she was gross.  
To the OP,  I don't think asking her to remove it would be something I would consider offensive :

Etiquette fancy word for paid hooker.

...it is a part of their identity and I would never ask them to remove a piercing any more than I would ask them to cover a tattoo.  
I would say either accept the tongue piercing as is, or move on to someone else.

I'd actually like to ask them all to remove their tattoos. Too bad it is not possible.  

Yeah yeah it's art.  Got it

Agreed

Asking her to remove it politely would not be offensive to most of us.  In my eyes, the session is first and foremost for my client to relax and be comfortable. It isn't about my preferences, it is about yours.

Unless it is painful to you.  For aesthetics?  Rude.  I mean, they put that piercing there to make themselves into whom they want to be.  If you don't like who they are, book with someone else.  Personally I am not turned on by tongue piercings, but have never turned down sex because of it ao I guess it doesnt turn me off either.

Same as tattoos on women.  They don't particularly turn me on, in fact I don't care for them in general.  That being said, I have not only had sex with tattooed women, but my current ATF is tatted.  It isn't for me to tell a woman what to do with her body and its decor or lack therof, its hers.

Now if it turns you off that much, there are tons of gals sans implanted jewlery, go for them instead.. Just don't be a dick about it.

Just say please.

I knew one provider who actually mentioned on her website that she would "remove" her tongue piercing if requested to do so.  Just keep in mind that some piercings, like nipples, close up quickly and are more difficult than a tongue piercing to take out and put back in - so you may get a "No" if you ask for those to be removed.  

But here's an idea:

Go see her, tongue piercing and all, and see if maybe you end up liking it.

I've pushed myself regarding piercings, tats, and other exotic features that you may find.  Most times I ended up enjoying it and therefore became a happier hobbyist with more catholic (and one time Catholic) tastes

I suppose you could ask her but more than likely she will say no. Tongue piercings and intimate piercings can start to close within an hour of the jewelry being out. More than likely she will not want to risk this. She may not be offended but may feel that you are wanting special treatment, as it sounds like she had made it very clear that she has this type of piercing.

If you Nicely ask her to remove her piercing, she probably won't have any problem doing so.

There are certain things that I look for in a woman when scheduling a date.  They are all personal preference and hopefully won't offend anyone.  

I am not a fan of tattoos.  I usually spend time with ladies that have none or one.  I have seen some women with several and the dates were amazing!!!!!!!  I do think it would be OK to nicely ask her to remove the piercing.  Decide if it is a deal breaker.

BTW, a civie I dated had a pierced tongue and gave the most amazing BJs!!!