I'm all-for harem members having fun with one another - I would in fact encourage it if I could. That being said, I'm only pro- that with girls who are already in the harem, or at least have been with the MC prior. Jess isn't with the MC, unless I'm misunderstanding, but Lucia and the MC are just starting out. In this case, if given the option (which I doubt we will be, given the opening scene), I wouldn't pursue Jess and would end things with Lucia. Going back to that opening scene... just because we know how it ends, doesn't mean we're okay with however which way it starts. It doesn't matter if Jess has the hots for the MC.. she knew that the MC and Lucia were starting to date, and decided that she was going to make a move on her anyway because - what, she thinks that's the way to win over the MC?
As much as I like poly-harem stuff, the way this one goes about it make me very uninterested in, and untrusting of, the two main LI's.