I mean there's no true canon for this because virginity/lack thereof isn't always so obvious. For each of them, I can see a few possibilities (based off their idolsonas and my characterization of them, for all we know the performers themselves are totally different offstage. I feel the need to reiterate that because virginity is a bit of a... sensitive topic lol)

Heeseung-->
option 1: totally virginal, and the reason he seems so intense/passionate is because there's nowhere for his sexual energy to go besides into his work/hobbies.
option 2: totally virginal, but is conversely hypersexual in an autistic way, which is why he comes across more experienced than he is. goons to hentai a lot.
option 3: has taken advantage of his looks to indulge in hookups without actually having to put much effort in socially. is sort of a garden variety early 20s slut, just a bit...lacking...in the human element...

Jay-->
option 1: virginal and very obviously so. is posturing whenever he makes references to sex or sexuality. panics if advances are actually made towards him.
option 2: headcanon courtesy of Ryan, he acts like he's a romantic and takes sex seriously and would never have casual sex and has a low body count. meanwhile the only sort of sex he manages to have are sporadic first dates where he immediately betrays his own values and ghosts the person out of shame.

Jake-->
option 1: virginal, but doesn't want to be and doesn't consider himself as such because he's technically had some non-penetrative situational sexuality activities with some of his friends lol. he's like THAT TOTALLY COUNTS?! (<--referring to the time he gave Riki a handjob)
option 2: obvious slut sending out slut alerts the way batman sends out his bat signal. what other reason does he have to act like that?! what's he even playing at?! nasty nasty boy.
option 3: not a virgin but nowhere near as experienced as he pretends to be. has had straight sex 4 times.

Sunghoon-->
option 1: virginal due to intimidatingly pristine looks combined with his dorky low-EQ personality and schizoid traits making him impossible to get close to. has his own gender/sexuality issues that prevent him from wanting to solve this problem right now.
option 2: the inverse of Jake, where he doesn't consider non-penetrative activities to be actual sex, so he claims to still be a virgin (cue heeseung looking at him funny remembering how he was gaggin on it last year) you can take this to its logical conclusion where, like the ancient Romans, he doesn't consider gay sex real or gay as long as he's topping. so he's technically still a virgin.

Sunoo-->
option 1: so there's no world in which Sunoo is totally inexperienced, I can see it in his eyes that he's seen dick and hole IRL before. he doesn't bring it up and gives a coy non-answer when asked, but everyone is Very Aware of the fact that he's sexually active, and it makes them insecure.
option 2: the only way I can see him being somehow a virgin is if he's never topped/bottomed before. So there's one cherry left to pick.

Jungwon-->
option 1: has had A Girlfriend. had sex with The Girlfriend and was bragging about the fact that he was gonna get laid it before it actually happened. he came back quiet and was like don't talk to me. it's a mystery what happened and he hasn't tried again since. The Girlfriend is nowhere to be seen.
option 2: obvious virgin is obvious. gets a thrill out of turning down opportunities. he wants to know that people want to have sex with him, but he doesn't actually want to go through with it because he's made it such a Thing that no matter how it happens it's gonna be embarrassing.
option 3: jungwon answers 'no' to the 'are you a virgin?' question, but looks humiliated. sunoo smiles smugly.

Riki-->
option 1: is technically a virgin but is all impatient about it because he thought he wouldn't be by now. that's why he was excited enough to commemorate his first blowjob with an ill-advised tattoo...
option 2: everyone ELSE was making a big deal out of it and it was really embarrassing for Riki... he tries to hide it when it does happen but it's super obvious by the look on his face and even if it wasn't, he looks like he got mauled on the neck by a biting creature.