Eh 'my house my rules' applies to what happens in said house. If the parents don't want boys coming over that's fair. But what Holly does outside isn't really any of their business and it's pretty controlling.
I mean it is controlling for sure, but at the same time there's a grey area where the owner of the house can impose the rules on outside behavior by making it clear that they don't want tenants who engage in certain activities outside. This grey area would be especially wide for parents/children given the regular dynamics between these two.
So i really don't see it as extremely strange that Holly's parents would arrange it in a way along the lines of "so long as you're staying for free in my house, i expect you not to act in public in a way that'd shame you, and by extension, us as well."
If Holly has a problem with her parents being this controlling, then she's had years to address that and move out. But as long as she didn't her claims of independent adulthood aren't very convincing and easy for her parents to wave away as evidently lacking.
She had to lie about who was going to the beach and got shit for practicing safe sex outside of their house. I've never heard of that controlling parents in my circles even amongst those that have lived with their parents past adulthood.
But how many parents have you known who were religious fundamentalists that believed sex is only permitted after marriage?
Like if Ian asked Perry to cover his rent, I don't think that'd give Perry the right to forbid him from sex. And as we see from Holly, those kinds of rules aren't enforceable.
Doesn't Perry literally kick Ian out for having sex with the girl Perry wanted to get on? This might've been unspoken rule, but pretty easy to foresee and effectively a limitation on "outside behavior" Ian was allowed to engage in, if he wanted to keep his place at Perry's.
Similarly, Ian is expected not to antagonize Perry, and this expectation isn't limited only to "while we're at the flat". If Ian is repeatedly a dick to Perry in public, this can eventually come back and bite him on the ass.
Like all regulations, those rules are only enforceable in the sense the person subjected to them gets to choose whether they value more breaking the rule or maintaining benefits they get from following them.
... And I'm guessing your referring to a Chad Ian. I wasn't aware his sexual prowess offered life changing decisions to that extent 
