There's no point where fertilization is not an issue, unless someone went under surgery to remove their uterus, or have other contraceptive measures such as IUD. Testosterone does lower the risk to get pregnant, but it doesn't eliminate it. There are trans men who got pregnant while on t.
So that's one thing. The other thing is that STIs are a big risk, and I think (not sure) that trans men on t are at an even higher risk, as t makes the skin inside the vagina more fragile (I can tell personally that I did bleed just a bit during vaginal play some times) and maybe there's a higher risk of infection. Not sure though.
Personally I don't do much vaginal, but either way I try to ask the penetrative partner to put on a condom... But I admit that I've found myself lost in the moment and going bareback quite a few times