Most Helpful Girl
It depends on the person you're with, but I'd certainly consider it cheating and I think most other people would too. It wouldn't be as bad to me as if they were cheating on me with someone my same sex, but it would still be bad enough for me to not want to remain together. I don't want to share my boyfriend with anybody else.
And there's nothing wrong with a relationship like the one you described in your update as long as all 3 people are okay with the arrangement.
Most Helpful Guy
This is most definitely considered cheating. While it will not feel the same to the partner of the cheater as conventional cheating, it will still hurt.
When a man cheats on his wife with woman, the partner of that man can compare herself to that woman and rationalize why he would do that. Now, I have heard that when a man cheats with another man, that the woman is unable to compare herself to that other man, since they are different genders. She can in no way "compete" with her partner's lover. That not only hurts, but is very confusing too