^^^^
First of all, JunJun is 19 so I doubt she's going to go "OH NOEZ! I can't lose to that!" and get preggers this instant.
Second of all, they have Yuko, Nacchi, Kaori, Kei, Yossi, Rika, and of course the leader and subleader, Miki and Ai, who are going to be a lot closer to them that Tsuji is/was, so I don't think there's any shortage of "good" role models. Plus we can't shelter Koha and Aika, they live in this world too and there are 19 year olds that get pregnant.
But anyway, it's not like Tsuji is in college, or starting a career. She's been a successful idol for over 6 years now and probably has a more comfortable lifestyle than most 19 year olds.