I too find it distasteful. Yes, under-age sex does happen but somehow if it's between two youngsters of a similar age it seems more acceptable. What I find worrying is showing a married man (albeit separated from his wife) sleeping with a girl of 14/15. Daz's comment was spot-on. Presumably if Cain finds out, Andy will be dead meat, but in real-life it would be a matter for the police with the man involved possibly being sent to prison. This isn't a portrayal of teen sex, but an adult being seen to take advantage of a child's vulnerability. (And, yes, I know Debbie is shown as the instigator, but the law on under-age sex is there to protect all under-sixteens, regardless of their characters.)
And I find Charity's attitude a bit odd, because of her own past. I'd have thought she'd have wanted Debbie to avoid making the same mistakes as she'd done, not encouraging her. Though I doubt even she'd be so approving if she knew who Debbie had actually had sex with.