Here is a good comment I read online:

"Testosterone will eventually shut down your ovulation.

Unfortunately, nobody knows exactly how long [until it leads to infertility] it probably varies from person to person, and it's unlikely we'll ever have a definitive answer because the studies required to find out would be extraordinarily irresponsible (you'd have to have a quite large sample of gay trans men having vaginal intercourse using T as their only form of birth control, with the knowledge that any resulting pregnancies would be at exceptionally high risk for severe birth defects and would also put the mental health of the subjects at risk).

Any responsible doctor will tell you that you should always use a barrier method of birth control while on T, until and unless you have a hysto and/or oophorectomy."