My boyfriend has been cheating on me with his ex(s)

My boyfriend and I have been together for 7 months. He's 7 years younger than me (he's 27). We just moved in together in the beginning of this month and have been talking about marriage - he discussed with his mom and sister that he was going to propose to me in a couple of months.

However, the very same day that we officially moved in together I discovered that he has been cheating on me with his ex-girlfriend (we'll call her Sarah) whom he dated off and on 4 years. This is a person who he has never been able to let go of. He also cheated on his previous ex (we'll call her Mary) with her.

At first he denied it to me, but then I got his sister involved who contacted Sarah and she told her everything (all too happily). Sarah forwarded the emails that he had written her (about 12) where he says he loves her and wants her back and can't live without her. How much he wants her. In addition to the emails, she said the've been sleeping together and talking for the last 2 months.

Once it was all out in the open and he had to come clean I asked why he did it. We have had a fantastic relationship from the very beginning - lots of laughter, sex and fun. Never anything bad. He said to me that he wanted to hurt her. That he wanted to make her trust him and then screw her over the way that she had screwed him over in the past (they had a VERY tumultuous and unhealthy relationship). He also said that it was just about the sex. She is 25 and has great boobs that he loves (I know this because I have found photos of them on his phone before). I have smallish boobs, so this makes me feel horrible.

He was going over there and having sex with her while I was at Yoga on Thursday and Sundays.

He says he doesn't love her anymore. He says he only wants me. He says he made a huge mistake. To show me, he has called her and emailed her and told her they are completely done. He has written a loyalty oath to me and sent it to his family and me. He has given me access to everything - his phone records, his email, his facebook, his phone... Everything. He facetimes me whenever and wherever he goes. He has changed his phone number and blocked her and his other ex Mary from his phone. I have his emails set up so that if she tries to write him, or if he writes her, that they will be forwarded to me. I literally have created a technology wall between them. However, I know that if he wants to, he can still just call her from work or go over there and see her. And that thought is killing me.

However, since I have access to his phone records I went through them all - all the way back to the beginning of our relationship. And he has been calling and texting both Sarah AND Mary (his other ex) the ENTIRE time. There are about 15 days in the last 7 months where there isn't a call or a text to one or both of them. He says he was just staying in touch. But what the hell?

I love this man. I really believe (or I did absolutely before) that he is my soul mate. He is everything that I am looking for in a man. And I had NO idea this was going on. I had some idea that he might not be totally over his ex, but I've been in those kind of relationships before and I know how hard it can be to forget about the person after you've had so much heartache. So I understand... But can I trust him?!?!

This all happened about a month ago (3.5 weeks ago) and I have resolved to forgive and forget as he is so adamant about it being a huge huge mistake that he couldn't find a way out of. But there is one thing that happened recently (since this all went down) - I saw on his phone that he had googled her and looked at the images of her. I just don't know what to do. I mean I still google my ex from time to time, so can I hold him responsible for such a small thing? Or is it just an indication that I need to wake up, smell the coffee, and let this guy go? I am so sad. I love him so much.

I, of course, called him out on it and since then he has called a therapist and is going to go to talk to someone about his issues with infidelity and lying.

What would you do? We live together now. I want this man and I don't want to lose him - not because I am afraid of being alone, but because he is who I want. But I don't want to be with him if he will continue to do this to me. He seems so genuine though. I want to believe him so badly.

What should I do? I am trying to make thoughtful decisions and not just react impulsively. I don't want to do anything that I will regret - either way.

Please help. I really need advice.