She's his first girlfriend so sti's not much of a concern, and pill more reliable?
And this is how it all bloody starts. Couple one, Katie and Oscar, are virgins and sure they're STI-free so they don't use condoms. They split up and go onto relationship two. Our boy and girl have already got used to easy, spontaneous sex on the pill without condoms and starting to put them on seems like a faff. Katie is certain she has no STIs and new boyfriend Tyler has told her that she's his second girlfriend and he says he was his first girlfriend's first sexual partner. He was, but what he doesn't know is that his ex-girlfriend started dating her new boyfriend before she dumped Tyler...
So they don't bother with condoms after more than a couple of times. Meanwhile, at Oscar's new university, the same situation is happening.
Katie and Oscar are nice young people, but neither of them like condoms, and Katie in particular finds the idea of insisting on a condom like saying she doesn't trust her partner's word. Eventually, Katie grows out of this and becomes more assertive, but not before she's caught chlamydia.
I am a member of thestudentroom forums. There was one thread on there from one young man studying a medical degree who discovered he had caught HIV after the results of the entirely routine blood screening done on all medical students.
Don't encourage your teenagers to avoid condoms.