Why would my boyfriend and a married woman like each other?

So, when I was looking at a late cell phone bill that I was about to pay, I noticed a phone number that was being called every day. I asked my boyfriend about it, and he said it was a friend that he met through a friend. But then I had remembered that phone #, when I had seen a text on his phone of a lady that was telling him that she loved him. He said that they were joking around. That's she's married and possibly has a 3 year old kid. He also said that even if he was into married women, he wouldn't want to with her because of all the drama that it would cause. 3 months ago, out of the blue, he texted me stating that he would feel too much guilt if he ever cheated on me. As naive as I was, I believed him. But his attitude towards me started changing, he was going out every weekend(almost) and stayed out until almost midnight, stopped holding my hands, or wanting to be around me. So, finally, on my birthday, he had told me that he likes her and she likes him, and that I should be glad that he's not out doing drugs, getting drunk, or f***ing sluts and that I was the reason why he started talking to her in the first place. Do you think that there's something going on or should I let him cool down until this passes over? We are sleeping in separate bedrooms now and he's been talking to this lady for the past 6 months.
Why would my boyfriend and a married woman like each other?
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