It is generally not recommended to pass urine immediately after having sex if you are trying for pregnancy. Sperms are released in the posterior vault of vagina from where they start to travel towards fallopian tubes where egg can be available. Inside vagina, pH is acidic and sperms die in acidic media. From posterior vault of vagina to fallopian tubes, this acidity is reduced and alkalinity is increased. Urine has a acidic pH which means it can increase the acidity of vagina during urination. This condition can kill sperms. Secondly, if you stand up immediately after sex then there are chances that ejaculate may ooze out from vagina. Some people have very good quality of sperms with high percentage of fast moving. These fast moving sperms start to travel immediately after release and can reach in fallopian tubes. In this condition, standing up and urination will not affect chances of pregnancy. But it is better to wait for 20-25 minutes after sex for start of any activity.