for the homosexuals.

I don't see how you've arrived at any conclusions regarding "cheating" based on what I've said since it has no relation to anything being or having been discussed.
If cheating is the act of having sex with someone other than your partner, without your partners consent, then any form of sex is cheating whether it's preferential or circumstantial because by it's nature it's the specific, voluntary, act that's cheating, not the reasoning or motivation behind it.

So murder might be a better example. You get a different sentence based on motivation and whether it was preplanned (preferential) or a crime of passion (circumstantial). In the court of law, there is solid evidence on what happened and what led up to it. Usually a full jury decides on the verdict.
Now take bi/homosexuality allegation. Let's assume you're not bisexual or homosexual with an encounter with the same sex. A court would look at what happened (sex) and what led up to it. The problem is the only person who can testify is the alleged person, plus anyone else was present who may be willing to corroborate with him. A jury has to decide on a bunch of statements about a "chance encounter" and a solid fact that the person has had sex with someone their own sex. Is it rational to trust the statements of friends and acquaintances of the alleged and say that he or she isn't a bi/homosexual? The issue becomes a matter of "do you trust the person" and a jury (us) wouldn't have an opinion either way, leaving us with the facts. Now is it fair for a bystander to claim he or she is a bi/homosexual?