Most Helpful Guy
For me its any form of intimate/romantic physical contact with another person, guy or girl. the only exception i have of this is if im dating a particularly kinky girl, i am ok with her having play sessions with someone else so long as there is no sex of any kind, with the understanding that there might be some nudity during the play session.
if you can compartmentalize thing, you can maybe forgive physical cheating in any form its a one time drunken mistake, and they truly regret it. if they are clear headed and knew exactly what they were doing, then not so much.
emotional cheating is something totally different. if they have emotionally given themselves to someone else, then chances are they have fallen out of love with you, and thats not something you come back from.