Is this using the quickstart option? Or from a full playthru? Also... If MC isn't dating Jenna, she still likes him, so if he offers sex, she will accept. so at some stage in the scene, you must have suggested / prompted it. Also, when a girl loses her virginity, she doesn't always broadcast it. When you lost yours, did you make a big announcement to the person you fucked?
Nothing to do with "broadcasting" virginity loss (though Katie and Jenna actually do so when you're on their path as you should know) , really if you imply that Jenna was liking the MC (despite the very low relation score and negative lust score at that point) enough to be willing to have sex with him just because the MC asks her on the spot, don't you think Jenna afterward would have talked at some point to the MC during their private conversations after the event about the possibility of further their relationship if she agreed to having sex with him out of the blue ?
Because that's the problem : her dialogues are exactly the same as if she never had any intercourse with him, so if she is liking him that much, that's even more odd.
Would have made sense with Porshe that is more of a one night stand type of character, but with Jenna that does not fit the character.
And actually all the characters are constantly throwing hints and banters related to it after having had sex with the MC in the dialogues during the game story. So that's probably why it feels very odd in that specific case , mixed with why she was willing to have sex so easily considering in this playthrough :