The Next Step is aimed at a teenage audience. In much the same way as Grange Hill included storylines about racism, drug addiction, teenage pregnancy, mental health and sexual assault forty years ago, The Next Step seeks to address issues that can affect young people, such as homophobia and misunderstandings about sexual identity issues.
I can see no reason why a programme aimed at teenagers shouldn't show a heterosexual couple kissing and therefore absolutely no reason at all why it can't feature gay couples kissing.