If you don’t regret it, then of course it’s not wrong. You obviously chose a moment when you were ready for it. Not many people are prepared at that age, but if you were, then good for you.
However, is it possible that maybe you’re looking back at your first time through rose colored glasses? Maybe it wasn’t that the sex was so great, but that you were so happy to be with who you were with? She was your first. We all have a soft spot for our first loves, a part of us may always be in love with them. It’s a strange thing. Doesn’t mean they were “the one”, but they have a part of us that we can’t get back – stays with you our whole lives. So maybe you’re looking back at her knowing how much you loved her and it’s just that you haven’t found the right person yet that will succeed her. (You will find that person eventually.)
We tend to over romanticize/dramatize significant moments in our lives especially when it’s with people we care about. If it was subpar, we can blow it out of proportion and say WORST EVER. If it was good, then that can sometimes be elevated in our memories to BEST EVER.
Either way, if you’re still comparing sex with your ex-girlfriend to current partners today…you might want to take a hard look at what you’re doing and why you keep thinking about her. Why was it the best sex you’ve ever had? What made it that way? What turns sex from good into great into amazing is usually how we feel about the partner we’re with.