I'm with you 100%. I never assumed she was a virgin, but there wasn't a single line of dialogue in the entire game, prior to her sex scene, that even hinted at her having lost her virginity, and several lines that could be taken to mean that she is a virgin. As you say, the dev could have gone either way based on what he wrote.
I honestly don't care if she's a virgin or not. What I care about is her ranting about her past sex life during a sex scene. It's a massive boner-killer and needlessly ruins the illusion of her possible virginity. Could have just left those 5 lines of dialogue out, and players could have decided for themselves whether or not she was a virgin or just choose not to think about it at all.
I am biased. I really like virgin LIs. Non-virgin LIs are fine, as long as their sexual past is never mentioned. For me, the best devs will make as many virgin LIs as possible. The second best devs will just never write anything about LIs' sexual pasts, one way or the other, and let the players imagine whatever their preference is. The worst option is to explicitly write about LIs sexual pasts. It adds nothing to the character or plot in 99.9% of cases and is just unnecessary extra information that everyone would be better off without.
Props to 
Juerhullycin for the very well-done rewrite. That rewrite of 5 lines turns the scene from a TMI nightmare character assassination to a hot kink reveal that doesn't even vaguely hint at any sexual past from Aine. It fulfills the same purpose as the original lines, but without making it sound like MC is just number 13 of 20 guys she plans to fuck today.