I'm really not sure why everyone is making a big deal about this, honestly. Kids and preteens do some really strange things because they are growing up and a lot of weird experimentation often goes on. Does no one know this or something? This is a common thing that kids and preteens do (weird sexual experiments like dry humping, especially boys) I once saw a kid getting a little too friendly with a starbucks chair and his mother didn't even notice.

I wouldn't care if my bf told me that, I WOULD care if he actually had sex with the dog though.

If I were your boyfriend I wouldn't have even told you that. Ever. So the fact that he told you that (too much info in my opinion) means he can be trusted.