Not really! My friends and I spank eachother a lot. It can be flirting though. I was best friends with a guy for a year. Then he ran up behind me when I bent over and spanked me. And that's what started it all. Now we've been dating for 2 1/2 years and have a baby on the way ^__^ spanking can lead to awesome things!
It means don't think too much about it if that is 'how he is' and definitely never date him. If it bothers you then you should ask him to stop. If it doesn't bother you and you liked it try to see things clearly here. He sees you as a sex object and is a flirt. Not exactly a good bf.