I've been with the same girl for a long time, wondering

It's a personal issue, as sex isn't a necessity
I can't read your mind

You're not missing out on anything. If you both truly love each other, that is the question. Sex is just one of the things you can do without anyway.

Not sure if this answers you question but, I met my fiancé at 18, I'm 21 now, turning 22 in a few months, and we're still together, still very happy, and the sex is still fantastic. We don't have as much time now with school and work, but we find a way. I've personally never once in these 3 years asked myself if I've missed out on anything (all that late teens early 20's bullshit like partying and stuff). I'm just not that girl. I think that as long as there's immense love in the relationship, good communication, honesty and respect, you'll always feel very fulfilled in your relationship. I do.