Why is it bad when two guys hug?

Here's what happened; my friend came over to my house for a sleep-over (yes, he's a boy) and my other guy friend was there, and the two of them hugged each other as a greeting while my mom and I were watching. It wasn't a big deal to me because we're all good friends but my mom got really angry and pulled me aside and told me not to let them do that. I don't understand why she was so mad about it since both of my guy friends are straight (heteroual, if you want to call them that) and it wasn't like a really pionate hug or anything sensual. Why do you think my mom acted this way? She didn't act that way when both of my guy friends gave me hugs, which I thought would've made her more angry than when my two guy friends hugged, especially when the hugs they gave me were more pionate.