I have dated a m2f, this was an interesting experience because although I am open-minded she did not tell me until we had been dating for a few months, which came as a shock to me, it did not bother me one bit, and I understand her hesitancy for wanting to tell me as not everyone is as accepting of the community as a whole, and I was a bit hurt that she hasn't told me prior, not that it would've changed my having dated her.
I also dated someone who at the time was still male and after we had broken up, the following year he began his transformation from m2f, we have stayed friends for 9 years