Cheating has no particular relationship with orientation - straight, gay, bi, or otherwise. It's not the bisexuality that's immoral here, it's the lack of consent. The genders of the participants make no difference.
A lot of straight people seem to think about bisexuality this way, and it has always puzzled me. Back in the 90's I heard a woman in a TV talk show audience during a segment on bisexuality ask (of a married bisexual woman on the show who were non-monogamous), "How come it's OK for you to have your little friend, and him [referring to her husband] to have his, but it's not OK for me and my husband to?" There was a stunned moment of silence from the four people on stage with the host... because of course, to them that had nothing whatsoever to do with sexuality and everything to do with consent in the relationship.