Why would a man flinch when a girl tries to hug him?

Then relax and wrap his arms around her? This happened to me while saying goodbye to him tonight and it was very uncomfortable! It was our first time hanging out alone.
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Hm... Erection huh? We were sitting in his truck so I don't think that's it but I love it if it was lol. Right before I got out we made another date so I know he finds me, at the very least, tolerable :p.
Why would a man flinch when a girl tries to hug him?
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