In the current public version you can hook up with Marco, Leo, Julian, and sort of with Owen. The upcoming Club Update will add hookups with several more guys.
There will definitely be more Leo, and Matt most definitely has a romance path. And Riley will absolutely get more sexy stuff. These first round of hookups are with characters who are open to hooking up with someone they just met. Riley would rather get to know you first... be he falls hard and fast. If you completed his route up to making out during the frat party, you'll be able to hook up sooner rather than later...