Why would a grown man get a hard on after hugging me?

So we're both 31 years old and have been friends for months. There's never been any signs of attraction from either of us until recently. We usually hang out with other friends but recently hung out together solo. Well we both know now there's definitely chemistry. Anyways, last night I sent him a message telling him that I was not expecting him to sleep with me I am not ready for that but I felt like he was avoiding me. We made plans and didn't happen because things kept happening whatever. So this morning he came over woke me up apologizing and said he was sorry and told me what happened but also told me that he wasn't avoiding me. He gave me an amazing hug and walked away but said I can't face you right now. He laughed so hard and said wow that hug just gave me a hard on!

What could this situation mean?

Why would a grown man get a hard on after hugging me?
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