As a Celiac myself, I would just like to say how amazing you are for taking her health into account. Depending on her age, the Guardisil (sp?) shot, (vaccines are all gluten free) could prevent her from developing HPV. As for kissing, she can actually get gluten from you that way. Kissing could actually be more dangerous than a BJ. Just make sure that you brush your teeth after eating anything with wheat in it. As for decreased immunity, assuming that she is following the gluten free diet and is otherwise healthy she is not at a higher risk of developing an std than anyone else. You still need to use condoms, but that is unrelated to celiac disease.

By the way, you can't get gluten from regular sex. Gluten cannot be absorbed through the skin or genitals. Read this page:
http://www.cdc.gov/std/hpv/stdfact-hpv.htm
It has a ton of great information.
Trust me, if you keep showing this much consideration to her needs, you will both do just fine.
good luck to you both