I think if you're completely in lust and in love with someone it's hard not to believe their bullsh!t. Not because you're a mug, but because you hold onto that glimmer of hope that, 1) they really love you enough to want to make it work, and 2) that depressing feeling that if you can't make this work, what hope do you have with anyone else?
Morally, I would always state that if I even had a sniff of my partner cheating on me, then I would dump him without question or explanation!!
Realistically, my heart sometimes defeats my brain and I find myself putting up with more than I truly deserve. Thankfully it's very rare though! :o)