this forum is amazing. thank you all so much for your input and insights.
some additional data points:
1. in all fairness, he told me on our 3rd date that he was still sleeping with his ex. he said that it was "tapering off" but still occurred. he said that he would be sad if I walked away from him, but would understand why I might want to. I saw a man who was trying to do the right thing (be honest with me), felt compassion and, foolishly perhaps, hope, and decided to go with my instinct which said "wait and see what happens." unfortunately, i did not say anything like "this is unacceptable to me" or "I am uncomfortable about this, how can we resolve this so I can feel safe getting to know you." he seemed so contrite about the situation, and so sincerely sad at the prospect of my walking away (his hands were trembling when he told me?!) that i felt that we understood each other's perspective. over the next several weeks, i would inquire about her being phased out and he eventually told me she was moving, but i guess i never sat down and told him after that how miserable i was with the situation. i thought it was *obvious* that i wanted an exclusive relationship. so i eventually told him a few week later, when he started bringing up his guilt and initiating conversations about feeling angst about his inability to give me what i deserved. but each time, i thought it was coming from a perspective of wanting to change things. now i just think he was simply stating that he felt badly, but had no plan or intention to change things. maybe bringing it up was his way of asking for my guidance. if so, i never got the hint and just listened and asked again for an exclusive relationship. he would repeat that he still needed to heal and didn't want to rush into a serious relationship yet again. i simply didn't know what to do, by then i had developed serious feelings for him. when he would initiate these relationship talks, i always told him what i wanted but assumed he also realized how badly i was being hurt. my bad for not being the communicator i should be...but i actually believe he was aware, but how could he know the extent if i didn't speak up. and i guess the reason i didn't, is that i perceived that he was unable to offer more, and that my only option was to walk. so i chose to continue to date him and see what transpired as talks of his ex seemed to be diminishing.
2. About a week ago I couldn't take it anymore. this was several days after my bday and i knew the instant that i saw him that he had slept with her on my bday...and, yes, he had texted me that night to wish me a happy birthday, sent me xoxo's. but i knew something was off when i saw him, so i asked and he confirmed. and i cried. and that is one of the last conversations we've had. he now knows i was devastated about his choice on my bday, that i am miserable with his lack of exclusivity, and that we have reached an impasse. he has asked me to make a choice that i would be happy with as he doesn't want me to be unhappy (in the "if you walk away from me, I will come find you one day and even if you turn me down, i will come back again" talk). i told him i needed time to know what i wanted to do.
so that's where i've been. detaching from him and deciding to either cut him out of my life, or date him over the next two months, non-exclusively (he will not budge on this, i asked) and see whether we truly are good for each other. an important shift is that i now want to know more about his moral compass...now i have concerns. and i no longer want him at all costs, i want what's right for me first. but you all are right to point out that i should have shown him how to treat me better and been more explicit about my requirements. it's just that it would have been over then and there.
so why have two otherwise intelligent people done this? it's my belief is that we struck a silent bargain at that restaurant on our 3rd date. he decided to downplay the extent and planned duration of his involvement with his ex (hoping that i would be there when it finally did end), and i decided to suppress my real feelings about his lack of exclusivity (hoping things would change towards exclusivity).
both deeply flawed courses of action. his infinitely more strategic/respectful of his needs. but now i have reached my limit and pulled away. the right thing to do is and always has been to say "call me when you're ready for an exclusive relationship." i've been gathering the courage to do this for the past week, and he knows something has changed. hence the texting me, the odd updates informing me that his ex moved*, and inquiries into whether i want to ever see him again. i simply don't know if i will walk away now that she's gone, but i do know now that i will be ok without him. i didn't how how it would feel to cut him out of my life, and it's felt good in a sense, but lonely.
there is no question that i love him, but now the question has been replaced with "who is he?" and, "is someone who let herself be a doormat, who suppressed her opinions even in a position to even want a relationship?" it's humbling to realize my role in all this, and realize that, even though i am loving and loyal, i might benefit from learning how to be loyal to me, how to be ok with some solitude and work on my communication skills.
* here's one for the behavioral scientists out there: after not responding to his texts for a few days while deep in thought over this past week, he finally sent me a text updating me on his ex, and telling me that he had dropped her off at the airport. he thought i should know. i guess i was glad to know she's physically gone, but stunned that he thought it appropriate to include that he actually took her to the airport.
p.s. pardon the typos, i am on a cell phone.