I find some of the responses here quite odd and hope they don’t have older teens.
The idea that having a sleepover accelerates a relationship unnecessarily, or virtually guarantees a teenage pregnancy…
They’re 16/17. This is probably their first serious relationship and probably their first sexual one. How this is managed by the parents can be a massive influence in how future, more serious relationships are experienced.
Acknowledge the depth of their feelings, that to them the relationship is serious and meaningful. Obviously cover contraception, sexual health, consent, respect etc. Lay down house rules - ok she wants to go to his but it works both ways, I don’t want to hear them having sex, see them wandering around in undies, be kept awake by them chatting and giggling. Respect the house rules. No weekday sleepovers in term time, no consecutive nights which might turn into having a lodger!
I do not believe allowing them the opportunity to spend a night together creates artificial intensity. Crikey, they just want to snuggle, watch movies, not have to get up and go when they get to that lovely sleepy cuddly part of the evening.
Knowing their parents respect them and their decisions, accept their boy/girlfriend, have expectations of behaviour and attitudes towards each other is very powerful. Respect and understanding breeds respect and understanding.
If in sixth form they are older 16 year olds anyway, and should be allowed the opportunity to explore relationships in a mature and responsible way.