As the others have already noted, in general, it's a "not good" concept. However, in small (as in, a few drops) amounts, it's not likely to be a major problem in the vast majority of cases. In larger quantities, it will definitely screw with the pH balance, and may very well mess up the natural microbial population, paving the way for possible infections, same as a woman can open herself up to a yeast infection (or worse) by douching too often. An allergic reaction isn't impossible, but I'd call it unlikely in the extreme. The main threat, as I see things, is the possibility of flooding her vagina with urine, then thrusting into her, thereby pressurizing the vagina, and pushing the urine (and more importantly - and dangerously - anything it might rinse loose as it passes through the vagina) through the cervix and into her uterus. This would definitely fall into the "*MAJOR* bad news" category - the uterus is a perfect home for damn near any pathogenic microbe that comes along, and if something gets a foothold in there, the bitch's death is a very realistic possible outcome. Overall, JUST DON'T DO IT!
If your fetish involves pissing inside your partner, and you feel you absolutely must, at least find one who can accept you anally, and do it there - The rectum - hell, the entire lower GI tract - is already absolutely filthy, microbially speaking, and the urine (whatever part of it isn't absorbed) will be expelled as soon as your partner defecates, or even sooner if his/her anus doesn't seal up immediately after you withdraw.
On the other hand, there's this factor to be considered: most men (most males, of any species, actually) have a *VERY* difficult time passing urine when they have an erection. Us males are built that way - because urine is on-contact fatal to sperm cells. The male who leaks urine into the semen he's about to ejaculate is a male who likely isn't going to pass his genes along to the next generation, so the ability for a male to piss inside his mate is going to breed (or more accurately, FAIL to breed) itself out of the gene pool in relatively short order. Millions of years of evolution have designed a nearly universal (at least among mammals) mechanism that makes releasing urine from the bladder during sexual arousal damn near impossible (and at least for us humans, often actually painful) so it's quite unlikely that such an event is going to occur without deliberate, possibly painful, effort from the male involved.