I would say it's fair to ask why, so long as there isn't any sort of judgement attached.
Some people simply choose not to and that's fine. It may not be how you and I live but it's a valid choice.
Some people don't wanna do it before wedlock. Also a valid choice but you do need to consider if that works for you.
Some also have traumas around the subject, like if their only experience was sexual assault or rape.
If it turns out the reasons are to do with simply being undesirable for one reason or another, chances are you'll notice those before you notice the fact they haven't had sex.