well - it's not 'following the rules' of sexuality is it? It's meant ot be boy and girl.
but with parents, they may be scared of not knowing how to treat them afterwards. So they use a defence. Not always the best one but they're just trying to come to terms with it.
My parents told me that if i said I was a lesbian, they'd be upset and shocked but they'd live with it. Mum's totally against gay relationships but she's not a homophobic.