neoseattle, do you have a T that you can speak with? You (said with kindness and understanding) have some big issues that you need to address in a safe environment.
I truly believe you need to work through all of your fears of infidelity and hurt (TRUST) first before you get into a relationship; otherwise, you are setting yourself up (and your SO up) for failure.
You need to learn to set boundaries and KNOW deep down inside with conviction what your non-negotiables are and who you are before getting into a relationship. That way you don't have to worry, feel defensive or be fearful. Otherwise, the other person senses/knows that and is being set up to hurt you. It's a confusing message (inside your feel fear and hurt and yet you are totally putting yourself out there).
In the meantime, listen to your instincts, and get out of there before you REALLY get hurt. IMO, he absolutely lied to you about the panties.
I do not mean to sound mean or harsh but it sounds as if you are a gumby girlfriend ... bending to your SO rather than just being who you are. I mean are you REALLY ok with a porn addiction or fetishes or infidelity or are you just saying that you be really "cool and understanding".
By expression your (valid) concerns here, but then almost canceling them out by expressing that you "are pretty understanding, non-judgemental, non-reactive, compassionate person who prefers to give folks the benefit of the doubt & believes people are innocent" until "proven guilty", & understand that we all have done things in the past we are not proud of....
This (to me) sounds like you are so terrified of getting hurt you are willing to be extra flexible and understanding and then go into detective and lawyer mode on someone else (using up all that energy), before you will even consider just trusting yourself and walking away. You KNOW what is wrong. Trust yourself.
Please listen to yourself. Let us know how you are doing. Take good care of yourself. Best wishes.
((((Rose))))