No visiters are allowed to stay overnight.. This is not suitable for me as I'd like to have my boyfriend of 5 yrs stay...
What the Landlord has said is clear - and unenforceable
if you have an AST. It's quite possibly a clumsy attempt at avoiding the risks of overcrowding. What you have said, on the other hand, is not
so clear. I'm going to assume you mean you'd like your boyfriend to stay overnight, sometimes, for the purpose of comfort, pleasure and self-realisation... not that you want him to move in as an additional person. If the former then you should be golden and I don't see that
a) the Landlord would even know or
b) care or
c) be able to do anything about it.
So you go get yours!

There shouldn't be even any extra wear-and-tear... even if the studio is furnished I'm going to guess the only thing at risk of overuse will be springs... in the mattress, sofa cushions etc... it's gonna depend on how athletic you are. We don't know enough to comment. We've all seen Fatal Attraction...