The Home Office have a campaign called ‘Disrespect Nobody’ which focuses around sexting, healthy relationships, consent, rape and porn.
This resource has a website full of information which could help you put together a session (or more!). In addition to this you can also find some discussion guides from the PSHE association which includes some lesson plans. Whilst these lesson plans have been designed for school use you could easily adapt them for your youth group. These 3 lesson plans cover: relationship abuse, consent and sexting.