I recently renewed my rental agreement and it says that the flat should not be used for any "illegal or immoral purposes". "Immoral" is an ambiguous term here. On searching the internet, it looks like this is common in most agreements and it usually means prostitution.
I don't think I will be under any trouble because of this. My neighbours have hardly seen my bring anyone and I don't think they would care or know that the girl is an SB or just a friend. But I am curious to know if they find out a girl is SB, are these legal grounds to evict me?
This is a standard phrase in all AST's to ensure that the landlord cannot be blamed for using his premises for the purposes of running a brothel. It means you cannot have a girl working there as an escort and have lot of guys coming and pumping her. It is totally OK for you to bring a woman in (paid or unpaid, FWB, fuck-buddy, gf, whatever) and bang her (respecting the neighbourhood peace and tranquillity of course).
"But I am curious to know if they find out a girl is SB, are these legal grounds to evict me?"
Respectfully, you are overthinking this. I speak as a landlord. As long as you pay your rent on time, are civil, keep the property in good order and don't cause a nuisance to the others on the street/block, you're every landlords dream.
Just enjoy!