You are maybe of the opinion that teenagers should first get used to social dynamics for some years without sex or maybe just not sex with older people. Without sex seems unlikely without locking them up, so if they're going to have sex anyway then why not with older individuals and wouldn't that even give them a better learning experience in terms of social dynamics and emotional resilience? From the point of learning it makes sense to facilitate inter-generational interactions. Then there's just no reason to block sex unless if you're a prude ;).
Many will have sex with each other, and I do not object on any prudish grounds. As for inter-generational sexual interactions, my judgment is that the number of bad relationships and the harm from them is much higher than the benefit from the occasional good relationship. It is a total cost-benefit calculation.
In my experience, just about the only people who strongly argue for allowing the adult-child interactions are adults like us who would be on the adult end of the relationship and perhaps enjoy it very much. When children list their grievances (or when adults recall their childhood grievances), nowhere remotely near the top of the list is prohibition of sexual relationships with adults (as opposed to blanket prohibitions on anything sexual, which might make the list).