Or she knew he was a virgin but didn't expect rejection? Maybe she thought he was like Taylor and had been a bit of an introvert with that stuff, and see how well someone reacts when they put themselves really out there, and get rejected.
What is more likely?
1. That Marc, a person known to be a good dude by his friends AND all the later times he is mentioned by Nikki, a virgin and not expecting it when he excused himself to go to the bathroom is followed by Nikki, a person who is KNOWN to be very forward when it comes to sex, is shown to go after guys, is known to have lost her virginity to the basketball team, and who is shown to think that sex is the most important thing in a relationship. But then she suddenly gets cold feet and is assaulted in the bathroom by the virgin. THEN after that, she lies to her Brother/Best Friend about the situation but continues dating the guy, never sleeping with him until they eventually get married?
2. The scene plays out exactly as written. What happened is exactly what Nikki said happened.
We have no reason to believe she lied about it, outside of not seeing what happened in the bathroom. And maybe the reason that people never see EXACTLY what went down is due to Sumodeine trying to get the information across while still trying to appease people for whom even seeing Nikki attempt to get with someone who isn't Taylor might be akin to ENTEEARRRRR! even if she is rejected. And her crying is because she is confused about the situation. Which would again be the point. I know you prefer the game modded without Grant, but in doing that you lose a key bit of development for Nikki. She has the hots for him, sure, she throws herself at him the first night they meet and end up having sex. But we also learn soon after that she thinks that it is going to go somewhere, and she is lying to herself about it afterwards. So yes, that reaction might not be overblown, people get legit shook when they get rejected. She thinks SHE fucked things up.