Honestly it sounds like you've got a few issues of your own and you've got some damaging perceptions of what a relationship should be. Not saying that this guy isn't slimy or anything, can only go by what you've said which is tainted with your perceptions and now what is the absolute truth.
-He shouldn't have to support her financially. Especially at her age. It doesn't sound like you know anything about his life before their relationship to be able to judge him. Just because he hasn't got a well paying job at the moment, doesn't mean he won't. This whole point should be irrelevant but you've seemed to make a big deal out of it. It's a damaging notion to think women should be provided by their male partners if they are straight. If there is support going on, it should be mutual.
About a month ago, Jess had gotten an IUD placed (She was fucking 18!)
-I've known some friends to get it as young as 15 (here in the UK). You're making birth control sound like a bad thing which is quite damaging to other women. They're human, and they have their urges, would you rather have them resist their urges until one day they can't take it anymore and have unprotected or partially unprotected sex? Because that's what happened the first time I had sex with my partner, we couldn't help ourselves and created a stressful situation. One that was easily remedied but was still a risky situation.
-Your friend probably put off telling you about her relationship because you'd react like this. You sound very judgemental, and you seem to act on emotions instead of rational thought. Of course I can't say that with any certainty, but you came across that way with the language you've used.
-I'm not saying this guy isn't a bad boyfriend, or isn't slimy, or isn't right for your friend, or isn't using your friend for sex. But I don't think you should, in good faith, say those things with complete certainty. You don't know and you sound unwilling to think otherwise and only find to satisfy your confirmation bias.
You probably need some maturing to do yet, as does pretty much everyone. No one should look at this this and think negatively of you. Just be more open and perhaps trust your friends more. I'm unsure if you posted here to your satisfy your own conclusions, but you can't be to controlling of your friends lives. People need to make their own mistakes and learn their own lessons. All these risks you've pointed out such as
she get's pregnant and he leaves her or that she's just going to get used like some sex toy and throw away her life
honestly sounds like you think you know what will happen and you know everything about either Jess or Marc. You don't. I can assure you you know competitively very little about your friends, such as everybody and their friends.
Just keep in mind that the way you perceive things may not be how they actually are. That is nothing negative towards you, I say that because you're human and that's how everyone is. Everyone perceives situations differently and has different ideas of what the outcomes should be, even when the situation has been concisely laid out for all to see. Just keep that in mind, it helps with a lot of things in life. Sorry for bad grammar, English isn't my forte .