Well, poly relationships no, because it's already very complicated doing all the choice branches for just one-on-one relationships. But that doesn't mean all those guys will want to be monogamous when you date. It'll depend on the guy, though I haven't decided on everyone's preference yet.
You won't be asked to commit to anyone for a while, though, so you can pursue relationships with multiple guys at once. That's why I have the "playing the field" sections in the guide, to help with that.
That's just romantically. Sexually, you can get with as many guys as you want, limited only by not being able to be in two places at once. I actually have a planning document mapping out all the hookup timing so I can make sure it's possible to get with everyone at least once in a single playthrough.