Why do guys do "hit it and quit it"?

I spent 4 months getting to know this man. It was a secret because we work together and I'm married. It was fun, lots of sexual tension and we finally had sex. Twice actually. Then poof... he's completely dropped off the face if the earth! Going out if his way to avoid me. I reached out to him to "end it" peacefully and with some class, but he won't respond. I'm told he's a "hit it and quit it" type. Why waste 4 months if that was the overall goal? Is it about sex, the challenge, torture... what?
Why do guys do "hit it and quit it"?
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