Why did my boyfriend sleep with an ugly girl during our break?

My boyfriend and I broke up in mid-December. We were apart for only a few weeks, after which time we were able to reconcile our differences. On our first day of contact after the break, he told me that he had slept with another woman "several times" after we split up. The girl that he had sex with is a friend of a friend (of his) and has had a thing for him ever since we began dating last year. Then, last night, I found out that he has since seen this woman - specifically, he was at a party that she was also attending, and he did not tell me. When I confronted him about it, he said that it just didn't cross his mind to tell me and that nothing had happened between them since we got back together.

At the risk of sounding terribly shallow, this girl is also HIGHLY unattractive and overweight while I'm an attractive, thin woman that gets hit on A LOT. I guess I'm just having trouble understanding why he would sleep with her, of all people. Is it a self-esteem issue?
Why did my boyfriend sleep with an ugly girl during our break?
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