'doing more than fine' isn't necessarily the most common way for a young virgin girl to describe 'getting drilled by a 20" phallus'.
I don't know, Nova seemed to enjoy it quite a bit. IRL, a virgin wouldn't likely be looking forward to her first time with someone who's hung like a particularly well-endowed horse, particularly with how, um, energetic MC and Nova were.
MCs in AVNs are almost always extraordinarily well-endowed, and even the virgin LIs rarely have any difficulty with his equipment (with Victoria in Cosy Cafe as a notable exception). Awe is common, attraction to it is common (women in AVNs tend to be size queens--see particularly Nancy, though she of course isn't a virgin), trepidation is common (Nova again: "Have you ever had sex with a girl? Is she still alive?"). But when it comes down to it, again with rare exceptions, they're down for it.
Moreover, the "young virgin girl" in question is very sexually attracted to MC (not to mention in love with him). She has at least some idea what he's packing after the event at her uncle's, but she hasn't seen it yet. But she wants to.
So let's suppose the prevailing theory (to wit, that Luna saw a sexual encounter involving her, Annie, and MC, possibly including others as well) is correct. Neither she nor Annie has had sex with MC at this point (though both are moving in that direction), she doesn't know MC's having sex with anyone (at least, not as far as we know), Annie's her friend but not her lover. She sees that, MC asks her about it, while they're still in public. What else would she say? Does she outright tell MC, "I saw both of us having sex with you at the same time"? At this stage in their relationship, I don't think she does. You can play with phrasing, but I think her answer is very much in line with her character and the relationship: she's going to be honest, but evasive. And we, knowing that every girl gets a threesome, and looking for clues, latch onto this.
At this point, of course, we can only speculate as to what she saw. I think the prevailing theory is correct in this regard, but I expect we'll find out in due course.
