Bisexual girl in a heterosexual relationship here.

Any sexual conduct with another person is still cheating regardless of gender if you did not have the explicit and prior permission of your partner. So yes, you did violate his trust.

If it is clear that your boyfriend would not be okay with an open relationship you don’t get to convince him otherwise. Trying and possibly succeeding to convince him is not only unfair and selfish, but will only breed resentment on his part. His needs and desires are just as important as yours, and standard monogamous relationship code of conduct doesn’t fly out the window because of our sexualities.

If you want to explore your sexuality and your partner does not want an open relationship then yes, you have to let him go first.