I think 16 is too young to be in a committed long-term relationship really. I think most people are not sufficiently emotionally mature for it and I think the opportunity cost in terms of other experiences you are not having because you are wrapped up with the relationship is very high.
We went to different universities so for 4 years we did a lot of things separately, seeing each other every 2-3 weeks in term time and more so over the holidays (lived in same town) which we had plenty of from university. This was before smartphones, texting and email. We had to use phone boxes and wrote letters.
I don't think you go into a relationship at 16y thinking it will be forever. It just kept going. Once we'd done university then thats when we got more serious in the sense of moving in together.
We were exclusive before that (tbh back in the 90s we had far less 'stages' to dating compared to what I see with dd and her friends these days) so we weren't going off with other people but we had our own friends, as well as joint (and separate) friends from home and lived our own lives. I could have cheated and dh never know but I honestly never met anyone else I'd rather have gone off with. He claims to have been the same and I have no reason to distrust him. I will never know and would probably rather not know these days, but I equally have no reason not to trust his word - in the same way he trusts my word.
I don't think there is anything I would have done particularly if Dh hadn't been around. In fact we probably did more in some ways than some of our single friends. We've had a fab life and are still enjoying it - together.
And for us it isn't that unusual. We have a number of friends who met their now husbands/wives at school, sixth form or university.
Would I have wanted it for dd? I'd want her to do what made her happy. She's 21 now so that time has passed. There are certainly some aspects of single life and dating I'd rather she hadn't have to experience - but so would she. But there are other aspects to not having a boyfriend at this stage that are making her next choices a little easier. So - it wouldn't have bothered me so long as she was happy and was still free to do what she wanted with her life.