I'm a man, I've been monogomous for 17 years.  I am recently in a new
relationship with a woman and the very first time there was intimacy,
there was no penetration, but my penis was in contact with her vagina.
24 hours later - slight burning when urinating, and an urge to urinate
constantly.  Doctor tested for STDs - negative.  Doxicycline cleared it
up immediately and I finished the 10 day course of antibiotics.  She
was tested for STD - negative.  We had intercourse approximately 4 days
after doxi was done. Almost immediately (within the hour) the symptoms
returned.  More antibiotics and it cleared again.  I had intercourse
with my other partner after the intial exposure, and before the
antibiotics were started and she has no symptoms of any nature.  I have
used a condom now for 2 months.  A condom broke and the symptoms
returned.  We are going back to her OB/GYN to talk to him about what
could be causing this.  My doctor, and a subsequent visit to a
urologist have found nothing that indicates I am unhealthy in any way.

Is there a bacteria (I assume it would have to be a bacteria because it
reacts to antibiotics) that I could be contracting from her that is
causing these symptoms? Would the symptoms show so quickly? Why hasn't
my other partner had any symptoms - or could she be asymptomatic?
Thank you.