What is Infidelity to you: For myself I view all cheating as infidelity however there are certain types of infidelity that bother me more than others. If I know my partner is coveting another woman, it destroys me, I might be somewhat conceited but when I'm in a committed relationship I want him to need and only want me.
I would prefer that my partner have a drunken one night stand with someone he felt no emotional connection to than to fantasize about another woman regularly.
As for myself I tried to be in a polyamorous relationship and I found my jealousy got the better of me. I simply cannot share when I'm in love with someone.
And in concern to forgiveness, if my partner was actually genuine, and the unfaithfulness was not premeditated I could forgive him.
I have myself cheated on a partner, it had been a three year relationship that was on and off and while we were 'back on' again, I saw a man I knew from university at an event and we just hit it off and ended up in bed together. I never told my partner, knowing it would hurt him more than it would satisfy my need to be honest and I stayed in the relationship for another six months longer than it should have lasted.