Why would a guy make a sex joke about you?

I mentioned that my back muscles were tight to a guy friend of mine and he made a joke about getting someone to ram me doggy style and how it might help loosen my back muscles up. Now the comment I had made to my friend about my back muscles was just casual conversation between us because we're good friends and I was no way implying he should do something about it simply because that would be weird.
Why would a guy make a sex joke about you?
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