If you follow Tara route, she said she doesn't want an monogamy relationship, at least for now.
Weeellll, that's not
exactly what she says.
She says she's not really looking to chain
you down in that sort of relationship. It's not so much that she wants an entirely open relationship, as much as she says she's okay with
you seeing other people.
The way I interpreted it (especially with her admitting that you're literally the first person she's ever been with) is that she's trying to avoid being too possessive, because she doesn't want to scare you away. So she's effectively giving
you permission to see other people (if that's what makes you happy) as long as she gets to be with you as well. You're basically the first person she ever fell in love with, and she's been carrying around a hidden crush on you ever since. For years she thought it was always going to be completely unrequited because you were already with someone else, and then suddenly out of nowhere and completely unexpectedly you reciprocate her feelings. No matter how cool she might act on the surface, deep down she's probably terrified that she's going to blow this chance she never thought she was going to get in the first place.
(And that's not even taking your own beacon-ness into account, where supernaturals all feel drawn to you. Or your position as the main character in a visual novel, which means you're a memetic sex god who can pretty much seduce half the people you meet and get people you barely know to agree to be in harems.)
Tara suddenly chasing after someone else doesn't really fit with her personality or her history. If she was that willing to look for fun or love elsewhere, she probably would have hooked up with someone else while you were still with Lisa. She's saving herself for you, even if you're not saving yourself for her.