The forum has no way of knowing what the last post you read was as it is not tracked

the last visit time does not indicate whether you have read a post, or even looked in a particular forum. Plus that is exactly how Active.asp works by default, it will show you posts since your last visit.
The two thinks are by no means the same, I can visit without reading anything at all.

yes, you could track it with cookies, you could create a cookie using the forum_id as part of its name and store the date against it, then retrieve when you want to use it

What kind of load would that put on a server for a busy forum ?

True, didn't quite think it all the way through.
Now that I'm thinking of it, maybe this would be a good use for some of the local storage options instead of cookies? In either case, you still have the fun of figuring out what to do if I read it on my laptop and am now viewing the forum on my tablet... to a programmer, it makes sense that the cookie/local storage didn't follow you from device to device; but to the average user, there would likely be confusion as to why it's telling me I didn't read it when I did.