Your semen poses a very low risk of infection on its own. It's usually not the ejaculation itself that causes the infection, the biggest risk to the female dog comes from having a dirty unwashed penis before entering her vaginal canal. After semen is released into the vagina, sperm immediately separates from the remaining ejaculate and sperms begin moving into the cervix and up into the uterus for fertilization in the fallopian tubes (if she is in heat.) Of course we all know that impregnation of a female dog from human sperm is impossible due to our biological differences. Once the sperm separates from the semen, it's common for the rest of the semen to just leak out of the vagina with the majority of the sperm heading for the cervix. Some of the waste semen will collect around the urethral opening after leaking out. It is absolutely necessary to take a female canine out to pee 10-30 minutes after sex, so that she can pee away the majority of the semen collected around the urethral opening. Not doing this can indeed increase her chances of getting a UTI.
A healthy vagina that has an appropriate pH balance for the species of animal it belongs to, has ways of eliminating unused sperm. The canine body should treat unused human sperm the same as it would for unused canine sperm. The unused sperm eventually dies and is absorbed by the body's cervical/ureteral mucus lining. Then attacked and processed by the canines white blood cells only to later be expelled from the body. Usually in the form of urine.
So, all in all, as long as you keep your dick clean, and let your female canine pee 10-30 minutes after sex... then, yes, it is usually safe to cum inside of a female dog and, no, a condom is not needed. *Note* you will want to disengaged from any sexual contact for 2-4 weeks after "standing" heat is over. If she likes sex out of heat, then you may presume sexual contact after the 2-4 weeks are up.
