I do not find the two choices, 'Cheating is still cheating' or 'It's not the same' to be exclusive.
Clearly any straying from a relationship is cheating, and if it was an intimate, monogamous relationship, then the trust necessary to maintain that relationship has been broken. I have never known any couples who successfully restored the same trust after something like this
That cheating happened with someone of the gender that I am not makes it different. That is, if my lover were with a man, I would wonder what he was providing that I was not. With a woman, I would know exactly what she was providing that I COULD not.
So, if I discovered that my lover, someone whom I care for deeply, was gay, I think I could be friends with her after the break up. Although discovering it by finding her in bed rather than through conversation would be a tough obstacle to overcome.