sends all visitors straight to the affiliate site."
You can tell .htaccess to send people to any page you like, it doesn't have to waft them off to a different site.
If you would like to send people to a page called say, subscribe.html you can do it. Just use a directory index. A directory index is a command that allows you to specify a default page that loads whenever someone types www yoursite .com (I've included spaces to prevent a 'click here' hyperlink forming). Instead of seeing your index file your visitors will see the page you want them to see.
If you put the line:
in your htaccess file and your visitors will go to that page. Once they're there you can ask them to subscribe, and then get the subscription form to take them off to your affiliate site when the submit button is clicked.