Is this an offense that I should easily forgive, or do I have a right to be upset?

Last week, when trying to sign into my email account on my boyfriend’s computer, it was already logged in. Before I noticed that it wasn’t mine, I saw emails about “Your craigslist ad looking for a dancer…” Curious, I clicked on them. Only to find emails responding to an ad that he had posted pretending to have a club and looking for strippers. He had asked for pictures to be attached… Not just face shots… Disgusted, I threw a fit. What made it worse was that he responded to the emails with an alias name (Jim) and played right along, pretending that he did have a club and asking for other pictures, basically. I wanted to break up with him right then and there. I don’t deserve that.
But, I didn’t. I basically said that I would eventually get over it, but that in the meantime, I don’t exactly trust him. Tonight, he gets in this huge fight with me about how I basically overreacted and that I caused so much drama and fighting that weekend. It was basically MY fault that we fought because I didn’t forgive him properly/fast enough. Apparently a two day turnaround isn’t good enough. My friends told me to dump him.
Also, he swore to me that it wasn’t his idea. Then I find out that it was. He says that he must have forgotten the details. I did forgive him but do not completely trust him still. I haven’t brought it up in a week, and then we were fighting and he brings it up as though it’s my fault.
Am I wrong to be upset about this?

Please, no personal attacks, just honest opinions.