The ssl protocol is IP based, so if you share the IP address of a sll site with other non ssl sites, all requests to thes other sites were redirected to the ssl site. So this is normal and not a issue with mod_rewrite.
There are two options:
1) Use a dedicated (different) IP address for the ssl site.
b) Add rewrite rules to the ssl vhost that redirect all https requests that are not for the sl site to the http version of the domain were the requests are for. I dont have a redirect code for this at hand, but I guess you should be able to find one with google or maybe someone else has one already and can post it here.