See, i heard recently that my ex's sister had been cheating on the boyfriend for a fair amount of time before they split up, even when under severe scrutiny.

what made her think it is okay? this question assumes that you think it is not okay, so let me include people who do think it's okay and invite them to explain why it is okay.

for the record, i've never cheated by my definition, but done things with people that might be construed as not-exactly-being-faithful. i mean in this instance proper, long term cheating.